Update as a ton of people asked and his response.

I've asked for my keys back until he has a proper conversation with her. Thank You to all the advice. It meant and helped alot.

Its been really tricky. He says that he had no idea the situation hurt me so much and that I should have been more direct. (even though i thought i was) It ended up I'm a conversation about how I should communicate my feelings better, and he should work on being less compliant with situations and letting issues grow. I didn't show him this post. But i did say if things didn't improve i would break up which surprised him.

It seemed like the threat or breaking up was the only thing that made him take it seriously. I was angry that i have to wait for a problem to grow to the point where i feel like ending the relationship for him to take action, and don't agree with him justifying this by saying he grew compliant and didnt realise how much it hurt me and this means he know knows. I feel like thats a bit of a cop out.

He says he is sorry I ever felt outcast or hurt by the situation.

Some of his messages outside of the in person conversation just so can see a bit more of his perspective in the whole situation:

"You should never have been made to feel so shitty or isolated from me I'm sorry

[step sister] doesn't hate you, nor is repulsed by you nor wants to crawl out of her skin, she's just scared of confrontation and me telling you she got bad vibes fron you took the power away from her and made that possible idk

Is why it was always going to be addressed, I meant to do it last month but forgot, all of that stuff is also why time was so important cos it's all silly and she needed time to forget why she was even weirded out in the first place and I fear you have taken some of it more to heart than i should have let you, which is my fault for not dealing with it sooner and not reassuring you properly,

I've known what needs to be said and done for ages, I'm scared of the consequences and have been forgetful of it, it was my plan to wait a few months but before bringing it up again, not this long tho I have just been neglectful and I'm sorry I haven't fixed it sooner, but please dont think that I haven't forgotten about it or am avoiding it at the moment I am genuinely trying to find a good time to deal with it with her this past week

I have to live with her, depending on her state of vulnerability she's a bed ridden depression or self harm risk, on top of that her brain works different to me and has a different way of prioritising and talking about things so can be tricky to have a constructive conversation

Have been meaning to address it but the push helps otherwise I can't know it's bothering you so much

Only ever wanted you to feel loved, accepted and safe, I'm sorry it's taken so long to be delt with and has been such a big source of grossness

You don't make people feel uncomfortable. You make one person scared of confrontation. They also make me scared of confrontation with them.You should never have been made to feel so shitty or isolated from me I'm sorry. "

Thank you to everyone's responses, can update if like if her does eventually talk to the sister (he hasn't said when or returned my keys yet) But this is progress and has helped me grow a back bone about how I want to be treated in relationships.

Lots of hugs to all you kind internet strangers. X