I feel bad for my brother in law. I can tell my sister is extremely manipulative and disrespectful to him, and generally bitchy. But he puts up with it. She comes to visit sometimes, and she tries to use her new tactics which she uses on her beta husband on me. Its new behaviour since she got married. I dont really stand for it, but it got me thinking. How is best to deal with it?

If it was a gf of mine i would leave them and not give it much thought, but the dynamics are not the same with siblings. Generally i just state boundaries calmly and if she acts up or gets vindictive i just leave the premises.