In it there are a boy and girl teen who are dating and their single parents meet and fall in love and get married and tells the 2 teens they can not date anymore- the teens of course couldn't stand this and got in trouble for being found in bed together- even though they were fully dressed and just snuggling- so the 2 teens go to the garage and turn on the car and commit suicide.

It's NOT Sin of Innocence for sure!

Possibly Surviving (http://www.imdb.com/title/tt0090106/reference), but they are not step siblings.