Writing about Forbidden Relationships

I honestly don't know what started this knack for writing about forbidden relationships.  Perhaps it's my belief in true love.  I think that we all have someone who is meant for us and only us.  With that said, life sort of takes us in crazy directions and our circumstances cannot be helped.  What types of forbidden relationships should I explore with this series?

So far I have..
-The taken man.
-The best friend's ex.
-The teacher/student.
-Boss/employee.
-Perhaps step-brother/sister??? Is that going too far? I promise I will not indulge in incest...that is just all types of wrong, but would it be wrong to write about a girl who falls for her step-brother?  What if they hadn't become step-siblings until they were adults.  How would this change things???