Your 12 year old neice is old enough to behave herself.Simple solution, you sister controls her when she's at your place, or she dosn't bring her over...PERIOD! I know how you feel, been there with difficult relatives and friend's children and yeah, I've upset a few parents, but see, I do have interesting things for children to do at my home, and sometimes they still think they can misbehave in my home. Then they deal with me, and I'm not pleasant when I have to say something...cause yoiu know, I SHOULDN"T HAVE TO. They don't have to be there. Stick to your guns, you tell your sister, if the niece don't behave, she takes her home, and you will help her another time. Don't yell, scream, have a fit, threaten suicide. You're being made sport of. Not acceptable, then again it is your place, and if you want to act that way I suppose it's your perogative.