Not sure what you meant by this? Obviously, she is nice to other people .
So??????
"Mean Girls" are masterful at appearing to be nice, WHEN it fits their agenda. It's all about maintaining 'control'.
It is all a part of being a controlling, manipulative, you-know-what.
You really need to begin to see this for what it is.
I agree with the previous posters that this looks like a very dysfunctional family situation...
Your older DD has some real issues.
And, I think that you do, as well.
The treatment and name calling towards your younger daughter is something can can NOT be excused or ignored, under any circumstances.
It sounds as if it has become downright abusive.
No question.
And, what would your husband do if you took the bull by the horns (as you should) and actually began to address your older daughters behavior. I am thinking he would actively undermine you.
This situation is NOT good, no matter what good things you try to say about your older DD and/or your husband or family.
Please consider getting some outside, objective, professional input here.