Hi, I'd like to help you with your question.
I can understand your concern about attending the shower as seeming like approval of the behavior that is going on here. It is sometimes very difficult in these situations to know how to handle the feelings that they create.
Although your nephew's behavior is not healthy, the baby shower is for the baby. You are helping the child, not his/her parents. Try to see it that way. This baby will be born into this situation at no fault of it's own so treat the invitation as any baby shower and bring something you feel will help that child get a good beginning in life.
You also may want to keep your relationship with all parties involved on good standing. You never know how this will all play out, and you may end up playing a very important role in the baby's life. If you get along with everyone involved, this will only help that child have an additional, level headed, and safe person to turn to.
I hope this has helped you,