Well you aren't really related... my childhood friend was in a similar situation! Lel lets see... how to word it... Her father died of cancer and her mother (who i cannot stand) married her (my best friend) boyfriend's father a few weeks later. So even though he was her boyfriend before... now she's basically -blam!-ing her step brother since their parents are married to each other... Crazy shit lol. I don't really see what my friend did as wrong since they were together before it all....but idk what to tell you though... this situation is a tiny bit different. And just saying, they aren't together anymore. It got a little crazy so if you go through with it... be careful with your heart.