My niece is very disrespectful to her Dad in front of us and the entire family. How do we handle this?

At our weekly family gatherings we see his kids curse at him, call him filthy name, tell him to shut-up, call him stupid, do the same thing to our children and everyone elses, break our house rules and so on and so on. This happens in front of EVERYONE and nobody says anything. They just ignore it. This drives my husband and I crazy! I don't want to stir the waters or offend anyone but how do I handle this. They are spoiled, disrespectful BRATS and he does everything for them.

I can't imagine any parent taking that, I am a mother and my children respected me, I requested it and gave them the same.
Is this your brother or brother in law... what ever the case .... it's happening and in your home. Your house rules should apply to anyone that comes over your threshold and I do mean anyone.
May I suggest a very civilized letter to the father and tell him that family gathers at your home are not going well at all, and with his help things can turn around 100%.
Express your love and devotion to them , but something must be terribly wrong in his family if he doesn't respect himself as a person and father. Tell him as nicely as possibly that you know he might be way over his head and maybe counseling for the family would indeed bring about this change.
Remind him in a loving way that your home is a loving and respectful home because you and your husband demand that of their children and themselves.
Lastly, tell him how you want to enjoy their family gathereings but when he comes with the children your rules apply to not only your family but to anyone who comes. That way you are not pointing a finger directly to him and yet you got your point accross in a loving way. Family problems are very hard to deal with and I applaud you for trying to get this one turned into a positive one. God Bless and I will keep your family in my prayers...........

If it was me, if it was happening to one of my kids, I would go to your niece and say, "I'm sorry, but we don't use that language in our family". And, then if no one stepped in to agree, I would take my kids and leave.
I mean, really. You don't want to "stir the waters"? How far will you go in the family to not "stir the waters"?
Will the child be allowed to hit your children? Smoke pot in front of them?
I mean, where is your cutoff on what you will put up with, and let your children be a part of?
Why would you want them to see the behavior in the first place. (You may say you son't want them to, but by being there, you are saying it's ok.)
I'm sorry, but I would offend. Big deal. So they get offended. I am so tired of the people doing the wrong things in life, blaming those who don't for being intolerant!!!

either talk to her dad about it when she isnt around or dont say anything at all....u cant really disapline someone elses kid....its not always that effective