Have you tried marriage counseling? Perhaps a counselor could help get your concerns across and help the two of you communicate on the topic of SD. You feeling broken and unable to talk to him about it can't continue - it'll kill your relationship.

I don't know your story so not sure how much custody your SO has.

Is she 50/50 in the home?  EOWE? or some other amount?

If she is 50/50 or less.. I think you will need to accept that her presence in your home will mean that while she is there.. you are going to be sharing your time with your SO.. Not saying you won't have a moment alone.. but she is there to be with her dad.. to be parented by him.. to bond with him.. all that stuff.  

I would be expecting that watching "adult themed" shows might be better planned for nights she isn't in the home.. since he seems to want to spend time with her while she is there.  I'm not saying that you and he could "never" plan an adult activity while she is there.. but if she is not in your home a majority of the time.. it seems reasonable that he would prioritize spending time with her for the brief space of time she is in the home.

If she IS in the home closer to 5050 to FT.. then he needs to do a better job of prioritizing his time and ensuring that he is not ignoring his relationship with you.. in favor of putting all attention on his daughter.. and not encuraging his child to be a little bit independent.

But.. ultimately, if you are unable to come to terms with the fact that you will most likely have to experience a sharing and compromise due to his other obligations to a child.. then it may not be the right relationship for you.  

OK a lot of questions - How often is SD there? How long have you and DH been married? When SD is there is it a rigid visitation schedule or is it all over the place so you never have a heads up of when she's coming? How many TVs do you have in your house and where are they?

I can tell you that I never ever make plans with DH on the days that SD is here. We have shows we like to watch together but we don't watch them on the nights she's here.
 

I made it a very clear rule without even having to say it out loud that SD was not allowed in our bedroom. Like I said I didn't even have to say it out loud because DH seemed to be on the same page about this. There is no reason they can't watch a movie together in the living room. 
 
Are you good at putting your thoughts down on paper? I'm much better at writing my thoughts and feelings and saying them out loud. I also found that every time DH and I would try to talk about things he would overtake the conversation,go on tangents and of course try to gaslight me so I wrote some very good letters in the past that really did work. The biggest thing was you never make it about HER you make it about HIM. One of the letters I wrote started with the words "let me be very clear. I don't have a problem with your daughter I have a problem with you and how you act when she's here"

I can also tell you that when I first met SD she was nine and the first couple years were very easy because it was like she was a little kid. The years from 12 to 15 where the hardest. Looking back I feel like DH was trying to hold onto her being a little girl and she was too. I remember one morning she came downstairs and she had to been 13 and in a baby whiny voice asked DH to make her some toast. It made me want to puke! As  soon as she got her period things started to change and in the past year things have gotten much much better. 
 

I know this isn't really about what show you watched, it's about him putting her wants ahead of your needs. Kick the kid out of your room, he can watch a kiddie movie with her in the living room. Set some boundaries for yourself if he won't. 

I made it VERY clear from day 1 that SD what not allowed in our room. That way, even if DH cancelled any plans, or wanted to do something with SD, I still had MY privacy. 

Also, why is a pre-teen even allowed to be in daddy's bedroom? 12 y/o is definitely way too old for that. 

Lastly, "safety" wtf does that mean? Unless she has some sort of medical condition that makes it impossible for her to be unsupervised (severe epilepsy is the only thing I can think of) there are ZERO reasons why you can't have privacy in your own home. Has he never heard of knocking of the door?

Sounds like your DH never taught his daughter what constitutes an emergency during the day/night. Bleeding, throwing up, etc. absolutely! Knock away. Thirsty, saw a bug, etc. a 12 y/o can take care of that herself and needs to be considerof others. We discussed with SD that if she knocks/bangs on our door at night, it needs to be for an emergency, otherwise, it can wait until morning. She's 10. 

WTF does your dh think could happen to his princess that he needs to keep the bedroom door open? FFS... She can walk, can't she? Knows how to knock on a door, doesn't she? WTF is he scared of? Maybe he's scared that you'll want sex while she's there and he's terrified she might hear. Gasp. Shock. Horror. I suggest you buy him a baby monitor so he can listen to her snore and you can close your bedroom door. 

Also, put your foot down about keeping her out of your bedroom. Make it uncomfortable for your idiot husband. Start leaving sex toys about. Wear skimpy lingerie. Get imaginative. 

She loves to watch Keeping Up with Kardashians and Japanese Anime. I am not into either. We do not watch tv in the bedroom. I want to watch what I want in my room and she watches what she wants in her room. She doesn't hang out in my bedeoom ever and I don't hang out in her bedroom.. we both need our privacy. Privacy is a form of safety. 
We do have a common interest of video games so we do set some time together to play Luigis Mansion together in the living room. It's what she and I do together to bond. 
 

She is my own kid and I don't want to watch kid movies in my bedroom at 9 o'clock at night. 

She's 12 and I'm assuming mobile.  I didn't like my SS being in my room either.  He had a tv in his room and could also watch TV in the living room.  I set that boundary with dh at the beginning, before SS came to live with us.  He had no reason or business in my bedroom.  That was my space where I could get away from him!!  Dh put up a small fuss but it had less to do with SS and more to do with other family in general.  He was raised in a household that didn't do privacy.  He was allowed in his mother's room whenever.  There was no privacy whatsoever.  He did respect it though and later came to see that he enjoyed privacy too.  SS was just fine not being given free access to my bedroom and your SD will do just fine too.  Your guilty dad husband needs to get over it.  

I get that she's his bio so he probably isn't bothered about sharing space with her.  My dh probably felt the same with SS, like what's the big deal.  But when it isn't your blood relative it's different.  I felt like my home was being invaded when SS moved in. I certainly didn't want him in my bedroom, my private space reserved for me and my husband.  I didn't and still don't feel that way with our bios.  They have both at one time or another slept in the bed with us, they use our bathroom, and neither one of us mind.  But they are both of our children.  It's different from steps, or some neighbor kid.  Would he want the neighbor's kid in his room watching movies or just being in his bedroom all the time?  That's how it is for you.  He might be quite fond of the neighbor's kid, but that doesn't mean he'd want his bedroom taken over by said kid. 

Not a SD problem.   She should not be allowed in your bedroom.  Two, you can lock your Bedroom door. I am sure she can text if she needs something.   Since most likely her phone is part of her hand.   
You can have some alone time.  At 12 YO. She can watch age appropriate movies. I am sure she knows a lot from her school friends 

Your DH is just a bad parent. He is trying to keep her a baby not a well adjusted teen 

I am still trying to guess why she is arriving at 9pm? DH works until 8pm?

Here is a new house rule.

KIDS GO TO BED, their own beds at 9pm. AT 9pm begins adult time.

Is that something you can tell dh?

About dh's niece? who passed away in her sleep. I am so sorry. Would it help if you suggested his daughter have a complete physical? WHY did she pass away and is the cause something that is genetic?

Finally, do you want to continue this marriage based on the way things are right now, not how you dream they would be.

(((HUGS)))) it is ok to cry once in awhile.

Yuck.  I'd play fire with fire.  Since he is ok w/her invading YOUR privacy, and she's ok stepping into your territory, turn it around.  When she goes into YOUR room to watch tv, take over hers. Go there, lock the door and watch what you want.  Tell them you're leaving them the matrimonial bed/room since it seems it's what they prefer and that you need as an adult woman, a private room.  I'd just move in there to make it even clearer.....move her crap to your room and take hers over.  Then make sure his family knows why you've done this as you need privacy and your husband can't give that to you.

Then start saving and move out.  

I agree with above. I would make my point in the most dramatic way possible to both of them. Since they are being ridiculous, I would be equally as ridiculous.

I would take all SDs things and move them in DHs room. Take all your things and move them in SDs room and say to both of them that they can share a room and SDs room is now your room.

I would refuse any intimacy with DH. I would let them both know that you are being supportive of them wanting to spend time together and you expect them to understand that you like privacy and your own space and the expect the same respect in return. This is now your room and your personal space.

I would not discuss the issue with either of them until they both came to me. DH agreeing that the bedroom is off limits and if he wants to spend time with SD it will be in her room. SD agreeing to respect your space and privacy.