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.