People change. Especially at your age - you were kids when you picked each other out, so finding out you're actually not a good match now is rough but also not very surprising. With that said, sometimes people (especially young people) say things they don't genuinely mean when they're very emotional, so I do have a "use your words" recommendation before you fall too far into the pit of despair:

If these changes are recent, it's probably important that you figure out where this is coming from for your own sake. It might hurt more but it might also turn out to be something solvable. You need to sit down with him and, while staying as composed and neutral as you can, discuss where things are and where you both see them going. If he's fine, this is over and it's time to let it go. If he says he's still in this with you, share that you're feeling like he's pulling away and ask where it's coming from. Share how hurt you were by what he said and find out why he said it - no one ever says things like that without there being some hint of something genuine behind it. Be as vulnerable, calm, curious, and accepting as you can be, and do whatever you can to not be defensive, and it should help him feel like he can open up better than anything else is going to. Discuss solutions or paths to solutions for anything that does come up. I will warn you that it's an incredibly hard thing to do and you never know what's going to come out of it - you could come out with both of you having personal growth goals and maybe some new communication strategies and you could be stronger than ever or you could find out that something has happened to end this relationship for good that colors your view of way more of the relationship than you're expecting or anywhere in between.

Best of luck - I hope you end up where you need to be in the end.

Unfortunately for you, she's not obligated to any particular relationship with you. She's an adult and she's allowed to be as close with or as distant from you as she wants to be, just like you're allowed to do the same with her. Do you reach out and try to connect? If not, it might be worth a shot if you want to be closer and want a real role as an aunt - connections take effort. But also, with 8 years between you, she was old enough to potentially feel like you were her replacement to her shared parent or other similarly resentful feelings, which aren't uncommon for half siblings raised in different houses, but at the time she was not old enough to really be understanding about the shared parent building a new family and that might still be where she's at with you.

Only you can really decide if trying to build a relationship with your impending nibling makes sense. I can tell you that you never really know where personalities will fall and connections will be made. But also, if you're going to bring any bitterness about the lack of connection with their mom into it, I wouldn't bother - kids can sense that kind of thing and it won't end well. Only try to connect if it's something you genuinely want to do and you aren't going to be resentful if you're not as close to their family as the cousins on the other side. If you try to get involved, get involved because you want to get to know your nibling as a person - you want to see them grow up, find out who they're going to be, support them through their hard times, and be someone they can go to if they feel like they can't go to their parents. Connect because you want to invest in them, protect them, and celebrate them. Not because you feel some weird half assed obligation to show up randomly because you share a parent with their parent - neither you nor they need or want that.

Only you can decide if this is worth it, but personally I'd move out regardless and reconsider this relationship from a distance; you don't deserve the way he's talking to you or treating you and there's no excuse for it. He's over 30, not 16, and he needs to grow up enough to communicate if you're going to have a life together. You were barely adults when you picked each other out, and you might've just outgrown each other. It happens. Or maybe distance will give you both perspective and you'll realize that this is really what you want. But either way it will be a change that cements how serious you are about things being in an untenable state and needing to change drastically before they can actually work.