Why do I hate it when my boyfriend drinks? I feel like he flirts with my sisters? Why do I I think this ? As I know sober he would never hurt me

Firstly, you could analyse if its a real problem or are you assuming things that way? Well, if it is a true story then TALKING THINGS OUT resolves most of the issues I am aware of. Slow and calm. Don't make very harsh or leading questions. Allow him to make himself clear and well understood. If you feel his drinking is over the top, then slowly, ask him how he would feel if you asked him to stop drinking? Talk him around some motivational info about how a relationship could be disturbed if people drink too much. Then ask him about what he has in mind about your sisters? When you say he doesn't hurt you when sober, then that is the best time to talk things out. But do it gently is my advice.

No one particularly like it when there partners drinks because they can do a lot of things they wouldn't when sober. If you think that he is flirting with your sister maybe you should sit and talk things through it most likely isn't doing it intentionally. Perhaps you think like this because your scared or loose him or because you know that when he's drunk he's not exactly in control properly so he could do anything. If you know he wouldn't hurt you when sober then maybe it best to sit down and talk him about how you feel when he gets drunk. This might help you understand things and it also means there is good communication going on in the relationship