dating a guy my sister liked.

My sister and this guy liked each other for a long time but never became official because she started treating him like shit, doing stuff behind his back and lying to him. It made me mad because he always treated her right...
Over the past year him and I have become extremely close, he's 24 and I'm 18.. I like him a lot, he recently confessed he has feelings for me too. We have had sex, been going on dates and just hanging around each other. He has been an absolute gentlemen, opens doors, respectful, wont use rude names when he's mad, he always goes out of his way to bring a smile to those he cares for and he always puts his family first...
Some women might say "there's a bro code" but I'm sorry I don't agree with that, I think it's childish. If you find someone who makes you TRULY happy, I say go for it.. Those who care for you should be happy for you, regardless of who it's with as long as they're being good to you. Everyone's entitled to their own opinions, so I'd like to hear yours or if you've ever been in a situation with a friends ex, etc..