I'm honestly very unsure about when they met myself. However, the two don't have to be mutually exclusive. The families could have known each other a long time, but it was when they started seriously dating that Joey noticed a change in the relationship with his sister.
I would believe this before I believe it was a production set up or that she intentionally forgot them. I could definitely see it being a subconscious slip, though.
Gina is 18, no? Could she not have taken two of her sisters?
I don't think I would have been offended by Melissa; and I'm generally on her and Jackie's side in all of this, but Dolores did have a point - Mel doesn't know what the others have or haven't set up for their kids. I think she should have kept it simply about praising Jackie's frugal ways, and not making comparisons with the others. I realize, though, that anyone could get easily frustrated trying to converse with Jennifer.