Wasn't the sister sat with the mom at the opening night, hana!?

Personally, they shouldn't have had Sly Stallone directing the film.

I have seen Staying Alive over hundred times and I have never seen this scene where Tony brings up Stephanie. Never happened.
On the OP question, Tony didn't keep in touch with anyone in the sequel because the studio didn't want to pay anyone from SNF, they had a couple of scenes for the father but in the end they refused to pay the actor so he was written out, they only brought back Tony's mother, this is the one thing that always bothered me about Staying Alive, they should have brought back for at least cameo appearances, Double J, Joey, tony's father, sister and brother, you would think the film would have been a lot better, who knows, at least they got some great new Bee Gees songs in SA.

If you accept that inferior film Stayin' Alive as the true sequel to SNF, Tony makes a brief reference to Stephanie iin there by saying she moved to California.