Is It Okay To Date My Best Friends Sister

Okay, so I have know my friends for a while now. We have got really close. And I added his sister on snap just to talk but it turns out she's really sweet and I would like to be more than friends. It's also kinda weird because he is a twin (I am a twin also) I just would hate for things to end up weird with him because he means a lot to me. I also asked him if it would be fine with him. He said ya he does not care he think it would be cool. I just want to know what everyone thinks Btw we are both 15 if that makes a difference