First of all, I am so sorry for you, your daughter, your family, even your SS. 

They say that “hurt people hurt people” and it’s really true. Often victims of molestation pass on their burden. At 8, he’s SO young, and exploring his sexuality likely, and should most certainly be taught never to do it again but I don’t think you neccesarily have a sexual deviant on your hands here. 

For context, around his age I started exploring my sexuality. My brother and I were playing a game and I suggested somewhat sexual contact (he was two years younger than me. I don’t want to go into detail, but no genitals were involved, and we were playing house). I can remember it vividly, that I didn’t feel like I was doing anything bad we were playing and I was imitating what I had seen on tv, etc. My mom caught us and I was in major trouble. We’ve never spoken of it since, but I also have never done anything like that since. It was explained to me that you don’t touch others without their consent, I realized I was doing wrong, and I never did it again. I’ve never told this story btw, it’s embarrassing.

I think you’ve done so right by getting him and yourself in therapy. You for your (justifiable) rage, and him to work through his feelings and behaviors. I am so sorry you’re going through this ::hugs::

edited to add: I wouldn’t leave him alone with your daughter or other children ever again, but I don’t think it’s wrong to try to help this child while also protecting yours and other children.

Risk of sexual abuse with unrelated male in house is far higher than other households.  I would move out. 

My SGS-12 became a convicted sex-offender at age 11 and is in a residential facility hopefully until he is 18 in order to get the intensive help that he needs.  The sexual assaults began when he was much, much younger (around 6 yrs) and initially were brushed away as precocious exploration.  Cousins and neighbor children all became victims through the years of this very bright young boy.  The cousins were removed from their families and placed in foster care since the parents failed to protect their children.

Not knowing anything about your situation, I recommend that you MOVE OUT.  I don't care how much you love your husband and what a great father he is to the 3 yr old, or whatever ... move.  Your little one needs to be protected and you are the only one who can do it.  Stay married, go to counseling, color lovely family portraits, send your daughter to other family, whatever you choose but get your daughter safe. Today.

You can PM me.

Edit to Add:  To some, it may seem that I am over-reacting, but it's just not worth the risk IMO.  The excuses, justifications, explanations (including that SGS had been assaulted himself) did not protect any of those other children.  You can make a different choice.

What did DH reaction to what SS did ? What punishment did SS received for his actions ?   First thing find a  a new therapist !  Your DD should see a therapist also.  Main thing if DH is not going to control his son then either son moves out of or you move out. 

Not all therapists are competent. We have a member on this site who claims to be a therapist but often gives very troubling advice, and sadly it sounds as if your SS got a dud of one, too.

Your first duty is to protect your daughter. If that means you must move out, do it. Sometimes love is not enough to keep.a relationship together.

That kid would not be in my home with my younger children.  PERIOD!  Even if he were my BK.  I would not serve my younger children up as his victims.

Your DH needs an ass wuppin too if he doesn’t put that kid over the arm of the sofa and take a belt to his butt.

GRRRRR!

These things were rare when parents had balls and would hold their kids to inviolable behavioral standards.

Some very close friends of ours adopted two young boys out of sexual abuse backgrounds.  The elder of the two mollested the younger and also several of his classmates.  Our friends surrendered him to a remote state residential behavioral modificaton camp.  They would not tolerate his abuse of his younger brother or his classmates. Heartbreaking though it is, they are parents of character and are applying the fullest consequence they can.

Since he is adopted the State has far too much oversite.  A regular butt wuppin would stop the behvior.  It may not address the root cause. But it is an effective behavioral control technique.

As an engineer I have made my living for decades optimizing the performance of assets and organizations.  The key is compliance to process and an effective accountability system.  A behavioral standard is no different than a business or equipment process.  When leaders hold the people in their organizations accountable for compliance to the processes that they are responsible for executing performance is continuously optimized.   Parents are leaders.  Family behavioral standards are processes.  Kids comply or they experience the consequences of non compliance.

Keep it simple.  Keep the emotion out of it.  Don't worry about the why of the chosen behaviors.  Worry only about compliance.  Adjust the standards as the behaviors mature and the behavioral performance of all involved will be continuously optimized until the children evolve to viable adulthood and launch.  If they then choose to not comply with the behavioral and legal standards that apply, their status is entirely on them.

Keep it simple. Stay consistent. Bring the consequences for non compliance.

IMHO of course.

Your husband still allows his son in your house with your daughter? Divorce him. 

Who is molesting SS8? He didn’t learn that on his own. 

My step kids no not to mess with my bio kids. I think i'd be in jail if my husbands older kids ever touch my babies.

I'm in dire need of help,

The very same thing has just occurred within my family. 
My daughter who has just turned 2 in October and just a month later in November my stepson was pushed to live with us. (Note- pushed as we weren't expecting it and only had a months notice of his arrival)

It should be noted that I am heavily pregnant with my second child and due to give birth any day now so the short notice of his arrival and presence into our lives was not all estactic or favourable on my part. 
Transitioning into our lives has been hard with a few mishaps as I'm rather strict and like order. 
My stepson sadly has lack of discipline so it's been exhausting to get him to understand why certain behaviour  and language is unacceptable. 

Moving forward to present day, currently the kids are off on half term and I thought to take them out before I pop to the  swimming pool. Being heavily pregnant with a toddler and 8 year shouting and running around isn't easy but everything had calmed down once they were in the pool.

(Now this little outing literally was just yesterday so I'm still very shocked. )
after the pool session and washing them I left the two in the changing room to quickly collect their clothing to change into, this was less than a minute as the lockers are just outside the changing rooms. 
NOTE : GRAPHIC
i return to a scene that will forever scar and imprint my mind of my daughter sat in the very same place I left her with her brother standing over her with his penis out, gripping onto her wrist and forcing my daughter to stroke him. I'm absolutely disgusted/mortified and scream and shout. The look on my daughters face and the look of being caught by my stepson... its indescribable.

I'm feeling all sorts of emotion especially letting down my daughter and trusting her brother. His immediate reaction is that he was joking but what 8 year old jokes around with his 2 year old sister such as this?!
 

My emotions are still raw at this point and I want nothing to do with this boy now and especially living around my daughter. I will never trust him now on top of other occurances but this is beyond help. 
my partner is trying to calm me and excuse his behaviour saying that he doesn't understand nor lacks the testosterone to feel anything but this is all just excuses I feel. In a short time since he's lived with us there has been too many incidents and this just tops it.

i have now given my partner an ultimatum however unfair it is but I can't ignore and continue living under the same roof as him with my daughter and unborn child. 
my partner is put into a terrible position but I'm not sure I care at this moment. 

honeslty just feel at a lost and what's excusing or stopping him when he does start puberty and full on attacks his sister when my back is turned? 
 

not sure whether I want advice or just an outlet but I completely understand what you're feeling. I just wish that this never occurred for either of us or any other parents but luckily we also caught it. 

Protect your daughter and move out.  Your SS did that at 8, he's going to do it again and again....especially when he hits puberty.   This is not normal for an 8 yo....he's way too young to be exploring sexually. Only other solution is to have him live full time with BM if she is in the picture.  Your DH probably won't go for that, but push for it.  He can visit his son outside of your house.  When he says no, pack your bags and protect your dear daughter. 

Last question...are you sure your SS hasn't been molested himself?  I'd be willing to bet on that.  Otherwise, he has serious mental issues to be exploring sexually at such a young age. 

I'm currently going through this. Idk how to get past the anger. My SS and husband are living separately from us. I miss my husband but I don't think I can ever forgive SS. 
 

I feel like I failed her. I just kept thinking if I had picked her up from my mother in law earlier it wouldn't have happened. Then I wonder how many times it happened before she told me. I pray she doesn't remember. She's only two. 
 

I feel conflicted for missing my husband. SS8's mom isn't in the picture. I think one of her previous boyfriends may have molested him. But that doesn't change that I can't even look at him anymore. 
 

I keep oscillating between sadness, anger, and disgust.