I agree with V trying shouldn’t hurt, the children are siblings and they didn’t ask to be here. The only person who should be uncomfortable is Dad and truthfully it should be up to him to do the work. As time goes on it’ll be easier for you to observe.
You can talk to her, but set up the expectations first with her that you aren't going to be her "sister-wife" or support her in specific ways. That's the baby daddy 's responsibility not yours.
Honestly? I would search my heart and make sure I didn't feel anything negative before I talked to her. To me it sounds very soon seeing as her baby isn't even here. Your babies are little and they're not going to start a relationship yet so don't rush your healing.
Sounds like you’re at least thinking about it. She may want to grill you about ‘baby daddy’ stuff or she may want to try and get the kids together (since they are siblings). Can’t know unless you speak with her. You can always try and if it doesn’t work out oh well 🤷♀️