She sounds like a narcissist....and your family behave like enablers.
The only way to deal with people like this is to go low/very low contact.
YOU decide when/how long you will visit and where
YOU decide on your boundaries and you spell it out to them
For example, i have a similar dynamic within my own family.
I'm the 'scapegoat/black sheep' and my narc sibling is the 'golden child' who can do no wrong.
Any time i speak/stand up for myself they try to shout me down.
Mum will be all "oh i can't stand the arguing"....but never tells the narc to 'shut up' or 'give it a rest'.
I've actually gone No Contact with narc sibling, because even with Low Contact she would use the basic 'hi' and 'bye' from me to get a dig in - usually by responding with 'hmmm' or completely ignoring me.
I refuse to have anything to do with her dc - because she uses them to play her narc/control games with me and i won't allow it.
Stop visiting your sibling and be honestly vocal about it if anyone asks.
If she's at your mums house and starts, totally ignore her like she isn't even there, literally ghost her.
If anyone else starts on you for it - tell them you won't tolerate being bullied.
At some point you may well have to go very low contact with the whole family if they keep enabling this dynamic.
You may want them in your life but you don't actually need people like that.
You are NOT responsible for how 'hurt' the rest of the family would be if you stood up for yourself........do you see them giving even a tiny shit about your feelings?
Google F.O.G (Fear, Obligation, Guilt) to get an insight into why YOU enable this dynamic.
I'm not saying it's your fault they behave like this, i'm saying people only continue treating you like this because you allow it by not having strong enough boundaries in place.
I came across this a while back and it sums up the narcissist perfectly
A Narcissist's Prayer -
That didn't happen.
And if it did, it wasn't that bad.
And if it was, that's not a big deal.
And if it is, that's not my fault.
And if it was, i didn't mean it.
And if i did, you deserved it.