There are times that a person who starts treating you like crap either is irritated with something you or someone else has done and rather then tell you they take the easiest route and "pay back" so I would bring it to their attention. If this is an ongoing problem and it happens frequently or in cycles then the only thing you can do is ignore/be nice and avoid
I've had to resort to cutting all ties with certain family members because they were making me sick, and I've gone up to a year without speaking to them. Pretty harsh I know, but if it's a parent like it was with me, I figure I've already endured enough of them while growing up. However, my dad is now bedridden and has been doing a lot of soul searching and he's finally seeing some of the things he has done. But it took a lot. He's had many illnesses you think would have jarred him out of his mind set, but he'll be bedridden for a year in 3 months, and it's taken until just recently for him to see things in the proper light. I feel sorry for him for his pain and discomfort, but I am also glad for him that he was able to finally see the human condition, which he wasn't able to do until it happened to him. Before, it was all ABOUT him. Not so much now. He has a lot of regrets, whereas before, all he did was blame. I don't know if cutting ties is an option because I know she just has you, right? My dad is in a veterans hospital. So he has not choice but to think.
There is only so much crap one can tolerate when it comes to family. You don't want to start a war, but you also don't want this family member thinking that you are a pushover and their behavior is acceptable. I usually let that person know just how crappy they are acting. Then I live my life and let it be.
I'm not real sure how nice you have to be. You just have to realize that it's not a reflection on you or who you are. It hurts when they treat us badly, they're the ones we're suppose to be able to trust that they love us and have our backs, it's not always the case though. You deserve to be treated well as you're a very special person, I'm not sure why people have to be like that.
CJ; I cannot say that I know how you feel, because even though I am going through similar situations within my family circle, I am positive, that your situation is unique. With that being said, all I can suggest, is the following;
Everyone has 'bad' days. Sometimes even 'bad' weeks. And people tend to hurt the ones they love. If, for example, a person has low self esteem issues, then sometimes they will make loved ones feel even worse, by harsh words or apathy. Even though it is clearly wrong to do this, people still practice this, all the time!
Here is something, you can try. When you are with the family, say something nice about him/her. But be sure it is something you truly feel about him/her. People need to feel appreciated. I hope this helps. (Speaking of appreciation, I truly appreciate YOU, for all the advice you give others!!!!)
I've actually cut myself a couple of times because of some of my family members. I treat them the same or a bit worse for a couple of hours but then I start being nice again because I know how it feels when someone is mean to you but I guess try to get strong and treat them the same or worse.