I know Janet had a sister and I think a brother was mentioned. I don't recall how many siblings Chrissy had, but I thought it was mentioned at some point. Anyone know?

Jack had one brother, Lee. Terri had a kid sister.

I always got the impression Chrissy was an only child.

I will agree that it was implied that Chrissy was an only child. So was her cousin Cindy. Jack's brother Lee was seen once and never mentioned anytime before or after. Upwards of two years before Lee's one and only appearance, we met Jack's uncle. Jack referred to himself as this Uncle's only nephew. I would say that this was inconsistent was Jack having a brother.