Married her stepbrother
Well technically, there is nothing really wrong with this happening because you are not related by blood to your step-siblings. They are just what came when your mother or father married someone new who already had children with a previous partner. But to marry them just to get back at someone else?
That doesn’t really sound like the basis for a good marriage, why did they feel like they had to get revenge on their grandmother in the first place? Was she mean to both of them and trying to keep them apart all of the time? Well, it seems like her plan backfired in the long run.