So  how about moving into your own place while staying married? Together but apart? Once your SS moves out (don't hold your breath), you can move back in with your husband. 

You have to put your children first. Their safety comes before anything else. When the time comes, you can explain to them that their big brother was a troubled person who needed more help than you could give him. 

Well, first of all.. your child does not need to be burdened with ANY of the reasons your relationship didn't work out.  Children don't need that kind of explanation.  They can hear that .. mommy and daddy loved each other.. but sometimes love is not enough.. or we were not compatible living together.. but blaming his brother for the breakup.. while that may feel like the push from you.. his brother isn't causing it... the inability of his parents to manage their own relationship is the reason.

I think you might want to consider winterglow's suggestion though.. what about living separately.. to give yourself some physical space from that environment?  Have you considered therapy for yourselves either?

I wouldn't worry about what to tell the kids as they get older.  They will figure it out themselves.  My ex was a charming, handsome, funny drug addict.  I never talked negatively about him and allowed visitation altho he didn't pay CS.  Both my kids experienced his unreliability and poor judgement themselves.  I never needed to say a thing.

When they're younger, we think we are shielding kids from family discord.  But my experience has shown me that my kids saw my ex crystal clearly and my steps saw BM, too.  None of them trash their parents but I know how they all think.

to questions as our foundation for raising SS.  Regardless of how young the kids are, IMHO they need truthful answers in an age appropriate manner.

In our case, it was just one kid. 

My DW did not walk away, she bolted and mostly saved herself and her baby from a shit show of monumental proportions.  SS had 16+ years under a CO and long distance visitation as he was growing up.

I walked away and took a do-over myself.  Though I blessedly escaped my first fortunately short marriage to my serially adulterous XW spawn free. It is one the the greatest blessings in my life that I did not pollute my gene pool with ... that.

SS-32 was 15mos old when his mom and I met. We married the week (5 days) before he turned 2yo.  He always knew the SpermIdiot and the SpermClan. We never bad mouthed or demonized them to him after a single incident when he was 2yo.  We learned that even when you think that you are speaking quietly and the kid is out of earshot, you are wrong.  After a crying "I love you and I miss my family" tear fest call to my bride from the SpermIdiot, we never again referred to the SpermIdiot or the SpermGrandHag by anything but their tilte and names.  "Daddy (FirstName) & Grandma  (FirstName)". After DW laughed raucously at the DipShitIots tearful cry fest, she asked him if he wanted to speak to his son.  DW called toddler SS to the phone he happily grabbed the phone and said "Hi Daddy DickHead!"  Fortunately his name sounds very similar to DickHead and when being spoken by a not entirely conversational toddler on a land line  what SS said was not perfectly clear.  DW took the phone back while DickHead expostulated "What did you call me?" DW explained it away at least well enough that DickHead did not harp on it.  His ganja addled brain likely did not retain that singular event.  SS was too young to remember it. Thankfully.

However, kids are smart. They ask questions. When he asked, we answered with the facts and the truth.  As he grew older and evolved his own perspectives on them and the situation of life under a CO, he started to put the pieces in place for the puzzle of the whole situation for himself.  The more he experienced, the more evolved questions he asked.  We answered then age appropriately.

When he was 19 he asked the big one. "Mom, what were you thinking getting involved with (FirstName)?" He already knew the answer.  She was young, he was nice looking and exciting.  She did not recognize at that time that he was a serial statutory rapist who predated on under age teen girls.  DW was 16 when SS was born. The DipShitiot was 23.

As soon as she gained clarity when he moved on to yet another 16yo when she was 17, she booted his ass out and set herself on the path to make a good life for herself and her infant son.  She left SpermLand for university after HS graduation and just after her 18th B-day and 2mos after SS's 1st B-day.

There were a number of interesting evolutions in SS's unfolding experience and perspectives on his life, on our family, and on the SpermClan.  For the single digit age range the SpermGrandHag was cherished by SS.  Then as he moved into and progressed through his teens he started to recognize that she is the root cause of the entire SpermClan shit show.  He was for the longest time of the opinion that without the Hag his three younger half sibs would have had a horrible life. As it turned out, because of the Hag his three younger half sibs have a horrible life.  As detestable at the SpermIdiot is, he pretty much has been only a rare and minor presence in the lives of his three all out of wedlock half sib spawn by 3 different baby mamas.  The youngest two are full sibs.  

SS is the only one not raised full time in that shallow and polluted gene pool.  His full time real life was with his mom and with me.  #2 was raised 85%+by the SpermGrandHag. The two youngest had almost zero interface with their BM so had nearly zero counter influence to the SpermGrandHag and the SpermIdiot.

#2 is on the dole and only slightly self sufficient. #3 is in prison. #4 is not far behind the inmate.  Our son (#1) is the only viable adult of quality and is a man of character, honor, and standing in his life, profession, and community.  He won the mom lottery big time.  The younger 3 all resoundingly lost the entire parent lottery.

Even the rageing banshee SpermGrandHag told SS when he was in his late teens that his mom was a good mom and the best person DickHead had ever dated.  It was the Hag who raged at my bride for being a bad "wife" and how it was her fault that her idiot POS waste of skin son cheated on her. Of course SpermGrandPa has been hosing around on SpermGrandHag for much of their marriage. Which is a well known thing in SpermLand.  My DW would not tolerate tht shit and flushed them all moving on to an incredible life.

The only one raised with any connection with the facts, the truth, and reality is SS.  He asked, we answered. As he progressed through his teens toward aging out from under the CO, he started doing his own research. After a visitation we would find him in our home office going through the several drawers full of Custody/Visitation/Support files, reading, listening to official court hearing recordings, listening to recorded telephone calls and answering machine messages of SpermGrandHag rageing at his mom, and reading the years of journals regarding SS's condition upon return from SpermLand visitation, MD reports and pics regarding his rageing butt rash so bad that his anus would bleed when he was cleaned up, puss filled welts on his rump, black sludge in his arm pits, elbow creased, crotch, and behind his knees, and body stench so bad that he would make a septic tank diver proud.

IMHO, kids need the facts in an age appropriate manner from the asking of the first question and increasing and evolving from there.

So, I advise upon the asking of the first question by your kids, answer with...."Your dad and brother were not healthy for me or you two to be around."  When they ask more, explain why. Answer honestly, factually, and with the truth every question thereafter.  Kids are kids and do not have any equal standing in the discussion. It is the adult's duty to give them the truthful answers.

As they grow up, kids need the facts to be able to recognize manipulations and to be able to learn to protect themselves when on visitation. They need the full history to be able to protect themselves as adults. Big bro will likely never be a quality presence in their lives and sadly they likely will have to recognize his toxic manipulations for the rest of their lives. Daddy being the source of the toxic big bro will likely also be someone they are fully informed of the facts, history, CO, etc... to protect themselves from.

It is sad that a kid has to experience this. But better for them to be fully informed than to be continually served up as an ignorant victim to the toxic side of their COD life.

I concur with the recommendation that you buy a place of your own where you and your kids live their childhoods. If you insist on remaining in a relationship with their father, make it clear that only their father ever visits and that toxic half bro has zero place in their real lives in your home.  Once the stench of half bro has moved on and daddy has clarity that half bro has zero place in your life and the lives of your children, then and only then consider reconciling.  

Hopefully, before that happens, you gain the clarity and confident to never settle for or tolerate a less than quality and worthy partner in your life.

I am proud to be that guy for my bride and my kid.  Who asked me to adopt him when he was 22. We made that happen.  That was nearly 11yrs ago.

Be good to you. Living your best life is the best thing you can do for your children and it is the best example you can set.  Toxic Klingons have no place when living that example.

IMHO of course.

Take the do-over and live well.

Take care of you and your LOs.