Is it okay to ruin my ex girlfriend's parent's marriage?
Oh wow, this can't be real. What are your reasons for wanting to ruin that?
They always act like they hate me
You should not ruin someone else's relationship. Even if you're not with your ex anymore, it's wrong to try to interfere with her parent's marriage to get back at her.
No one will know.
I think you need to stay out, how do you know no one will know? I'm sure someone can find out, please be more respectful. Then you will be in big trouble with everyone.