Dad and new stepmother want 15-year-old son to sign "family contract" to "accept" their new family and waive right to objection: '

Family is… family—something that sort of just happens. Springing naturally through strong connections by blood, intentions, reciprocal care, or shared experiences. 

(Insert any of a number of quotes from Don Toretto about not turning your back on family and/or something about family being the most important thing here.)

It's not something you can just force into existence, hoping that by sheer willpower, you'll convince the other members of the family you're trying to build that they should also regard the relationship as such. Ironically, although marriage creates a family in a legally binding sense, the essence of the family couldn't be any further from such. If we're being specific, marriage is really just the legally binding act enacted between the two parties once the family has already formed. 

This is always tricky when it comes to step-parenting and composite families coming together; you can't just tell your children—especially those who are old enough and teenagers—that they should feel a certain way about the new arrangement that your relationship has thrust upon them. 

That was the issue at hand in this thread, shared by a teen to this online community wondering how he should navigate his father and new stepmother's complex and unusual request.