Step-family

Okay so this isn't for me but my cousin was asking me for advice about this  and I didn't know what to say. So for this to make sense in gonna write this in my cousin's Point of view. 
Okay so my mom got married then later divorced to a guy. The guy already had three kids before marrying my mom and his two daughters ended staying with us to live while the son went to go live with his mom. Now a couple years ago I went with my step sister to her cousin's house just for the summer. I ended up having a major crush on her cousin and he liked me too. Fast forward to now, everytime we see each other we always flirt and end up making out. Thinking about it now I'm wondering if this is wrong? I want to date him but is that okay? Would he technically be my cousin?