my attempt at this!

CW: user got cheated on by Mary's sister.

Your ex cheated on you multiple times and you got sick of forgiving her, so you two broke up. That wasn't enough, you were still hurt by her action. So you decided to try and hurt her in return, by dating her sister.

Initially there weren't any true feelings towards Mary, but as time went on and you got to know her, you realized that you made a bad choice all those years ago.

IMPORTANT: user is 24 or more, as she's 29 and i don't feel comfortable with user being younger.