OP, I assume you were only kicked out of the house temporarily until tempers cooled off. Unless she has dementia, Mom is perfectly capable of handling an argument with your sister and does not need your help. In the future if such an argument kicks up again just leave the room. If you aren’t there you cannot get dragged into the argument.

either walk away or go in knowing the consequences... I get defending yourself, but when it comes to parents, being the third wheel in an argument tends to make you the villain. I know from experience. SO as hard as it is in the moment, consider how worth your stress it's going to be. sometimes you just have to swallow your pride and walk away.

You might want to invest in a thesaurus to look up some synonyms for "nasty."

Now you know who the favorite is. since your mom reacts that way just walk away the best that you can because all the fault will continue to fall on you.

"Naturally I got nasty back" come on op dont take the bait that easily.

So you're nasty, so what? On the bright side now you can refer to Janet Jackson as Ms. Jackson