Brother in law is a jerk?

He fulfils something in your sister at the moment....don't say anything to her about your feelings towards him. In time you may find she is not as happy as she appears to be. If you continue to have good relationship with her , she will feel free to talk to you about any problems she is having with this rude / obnoxious guy. Try find a way to spend time alone with her...maybe take her out for a meal / shopping / coffee anything to get her away from the house and him....but i don't think it would be a good idea for you to criticise her mate...it might just make her build a wall between you.