My sister is due in October.
Well I'm sorry to hear that you aren't close to your sister. I don't now what that's like since I'm really close with my sibling. I feel like you have given the olive branch to her and she probably is so self-involved she doesn't realise the damage that's being caused. But if she's said things like she doesn't want you in her life... I would want to back away from her as well. I'm guessin she contacted you about the news. So you can send a message back saying you are happy for her. I wouldn't necessarily send a gift right away. It's one thing if she invites you to visit or for a baby shower but I wouldn't send anything at the moment. Wait until the child is born. Then send a small card & gift. I mean family can get annoying but you know if you don't send a gift & a card she'll use it against you for life saying you never acknowledged the child. Hopefully the child & being a mother will soften her? I'm assumin it is her first kid.
When/If you receive a birth announcement or phone call of the birth, send a congratulations card to her. That is all that you need to do.