My niece was born to a teenage mother (18), and my brother, who at 23 wanted nothing to do with her. In fact he never even held her until she was 4 months old. Our situation was rather like the OP's, in that we did not know a baby was coming until the mother was 7 months pregnant. My brother refused to tell my parents. My parents found out when the mama sent them a baby shower invitation with a letter inside it telling them it was for the granddaughter they did not know existed.
We did not know this girl at all, but did our best to get to know her and her family. Long story short, she lived with my parents until Ella was 8 weeks old, then left. My parents have been raising Ella since (adopting her when she was 4).
The point to my rather rambly story is that your sister needs to set boundaries now, before the baby is born, and stick to them. She will need to rely on your family, and extended family if possible, but make sure other grandma knows who is in charge. It's fine if other grandma wants baby things at her house, but don't throw a shower for sis and then keep her gifts. They are not hers to keep, and I would not allow it if I was S. I would simply pack them up. Really, if I was your sis, I would not even go to the shower, as PPs have suggested.
Good luck and congrats on being an aunt. Even if it is not the best situation, there will still be a precious little baby coming who will need lots of love, especially from an aunty! That was one thing I had a hard time with. While we were not thrilled with the whole situation, it was not Ella's fault, and I was rather excited about being an aunt.