My wife and family hate each other...?

This has been an ongoing battle for 10 years now. My wife feels that my family continually does thing to make her feel like and outsider or just to piss here off. At first she and my Mom where best friends, but a fight over whether to draw names or to give gifts to everyone (which we could not afford at the time) started the down hill battle. Ever since then whenever they do something to make her mad, she leaves it to me to handle it. So I would, but in her eyes never stern enough. She feels that I favor them over her and that I think it is ok for them to walk on her but I can't stand up to them. I admit I am not a confrontational person, but felt what I said each time was enough. It eventually came to her staying away from them (which is hard to do since my Mom lives in an apartment out back). The latest event started a heated argument over not keeping the gate closed so our dogs would not get out and my 19 year old niece slapping my wife along with some nasty words from my sister. Now at my wife's request and my agreement, my niece and my sister from the property, which means they can not visit my Mom. I believe I should stand up for my wife, but what do I do about the family I also love? Now I am on the brink of divorse and feel like I am getting ripped apart from both side.