My cousin and I aren't blood related e.g. his mum is not my Aunty. We were bought up together as young kids and have always got on really well, when I was younger I used to look up to him and he always used to be very protective over me and his sister - the same age as me. Recently He stayed at Uni with me because he had a job down the road we were both really drunk and ended up kissing. Is this really wrong? I don't know how to deal with the situation.
Still, sometimes people judge -- especially family.
The riskiest thing might be that once the infatuation is satisfied, he will still be around.
Anyway, your families could actually be pleased, you never know:)