Firstborn

The movie "Firstborn" 1984

1984 - Teri Garr, Peter Weller, Christopher Collet, Corey Haim, Sarah Jessica Parker
Because he's the oldest, Jake has been the man of the house since his parents' divorce. When Mom starts seeing Sam, who always seems to be trying some new way to get rich quick, and declares he's the man of the house now, Jake puts up with it until he discovers Sam's illegal activities.

Trailer "Firstborn"