I like the idea of you confronting the bf with what you know, more than being the one to tell your sister, what you weren't told directly, yourself.

If you could get word to the dancer, to tell you sister, herself, that wouldn't be a bad idea.

I hear your worry about your sister, financially. Her bf, has 1K to blow on one event, at a time. That's quite the little bankroll he's got floating around there.

If his name is on their childs birth certificate, she could claim for child support. There are means for single moms to work on getting back on their feet, financially speaking...if she did need to kick him to the curb.