Late response, but it seems you actually do need some help with this and I've decided to start checking the forums more.
I used to have a similar problem, (the main difference being that I am close to my sister) and while this sounds assinine in nature, it gets the job done...
First, find out the real issue behind it. Is it your mother feeling insecure about your sister's behaivor, or is it a sense of feminism, possibly a guilt trip? Find the deeper context.
Agree to dissagree with your sister, sit down with them, discuss what the problem is, maybe they have emotional issues, maybe they're just a plain out *****? You won't know until you ask them. If they try to pull some kind of BS while you are trying to discuss the issue, drop it, as they aren't going to tell you.
See if there's something in particular that causes these problems. Is it a tone of voice you use, or possibly something you say? It is tough to admit in a situation like this that we are at fault, and I am not accusing you of such. I'm simply saying that you should look to reassure yourself you are not.
From the sounds of it, you don't have high chances moving out, or if you do it would take at least a month or so. If you cannot come to an understanding, or at least mutual respect, than that's the main problem. Sub consciously your mother, or sister feels that they are able to use something to their advantage to trample their negative feelings out on you, and the other is following.
This sounds like a bad idea, I know, but if the problem is not solvable, then a better alternative is to prove your "superiority" in that you are not some rag doll they can throw around as a metaphor of their own personal problems. This does -NOT- mean be an ass. What this means is to say you are better than what they throw at you. Don't lose your temper, if a fight feels like it is brewing, simply say to yourself "I'm the better person, I'm walking away from this sh**."
If they try to take the problem to you, then show them you mean business. Show that you are not afraid to stand up for yourself, and what you believe in. And do -NOT- under -ANY CIRCUMSTANCES- back down.
This next piece of advise might sound like a bad idea, but sometimes a good ol sibling a** kicking solves the problem. My younger sister used to constantly get away with everything under the sun, and one day while my parent's were away, they tried to pull similar stuff. I ignored them, they decided to throw a punch, and half a minute later I had them in a headlock in a position in which they couldn't move, screaming at me that they hated my guts, blah blah blah. I got a family shitstorm out of that, yelled at, and grounded for a month, but you know what?
They stopped. They got the picture that I was not going to put up with them, and they stopped bugging me like that. We get along just fine now, and they understand that I will react. Sometimes, people just need to get their butt kicked, guy or girl, to figure out what they are doing isn't right.
Ohh yeah, and think happy thoughts. It'll do wonders for you, trust me I know

(law of attraction and all that quantom physical mumbo jumbo actually has some truth to it)
Anyways, sorry for posting late, and for my slightly vulgar language. I tried to keep it at a minimum!