When something bad happens in our family. Like a death or a funeral. She's nice for a few days and then when it's done she starts being g a BITCH again. WTF is wrong with her?
It makes me not trust her at all.
I don't believe ANYTHING she says.
I lost respect for her.
I lost trust for her.
Probably will never speak/associate with her ever again.

calm down op.
shes a woman, a female. they have weird hormones, are incomplete (if without a male partner) as they have monthly or so bleedings.
but she is still family, your family.
maybe swallow your pride and when shes a bitch next time, you may just calmy approach her and hug her and tell her that she is your sister and even thou shes nagging or bitching orwhatever, you still love her. that will probably defuse her.
when shes defused you may ask her why shes behaving like she is