Anger at having to deal with this

[daughter]

I'm the former SG "dutiful daughter" who was supposed to be subservient to perceived demands and attentive to emotional needs of my npd-mom and GC "princess" nsis, my only sibling. I was required to tolerate their disdain and disrespect, to quietly endure their pompous grandiosity and selfishness. I was frequently criticized for "not doing enough" for them, because I was worthy only for my usefulness, and for "being jealous" of them, because they dumped their emotional baggage in me.  What helped me to persevere, to cope, was to understand these were their personality flaws, not mine, that their pd-behavior reflected their problems, not mine. I would remind myself that nsis constantly asserted her superiority, because she felt insecure and unhappy. Ditto for npd-mom. Extremely enmeshed, nsis' life and experiences are felt by nmom as literally being her own. There's zero emotional separation; if nsis stubs her toe, nmom cries-out.

Nmom also replayed her vicious childhood rivalry with her only sibling, my SG aunt, by foisting same toxic dynamic upon her daughters, to compel us to not "be friends", to explicitly destroy whatever nascent bond we had. This was so obvious that ndad, in his typical passive-aggressive manner, would often blandly remark upon "your mom's done all she can to wreck your relationship with your sister", and "your mom doesn't like you", as if this was normal family dynamic. Yes, it's a big blow to hear this, to see this dysfunctional family dynamic normalized, to see my anxiety trivialized and to be constantly berated for my supposed shortcomings in "serving" my FOO family.

Ndad and nmom frequently stated to me, in plainest language: "it is what it is, and we won't change, so you better just accept it", meaning their bad behavior and emotional abuse directed at me, SG.  According to nmom, all families pick a SG child, as her perverse justification.

NC, for me, was a belated but much-needed, choice that addressed my needs as finally my 1st priority.  I realized I couldn't "fix" my FOO, that they had zero motivation to change their expectations and demands, and no desire to be "nicer" towards me.  Sometimes we need to "accept situation as-is", meaning we step away, literally stop engaging, in effort to effectively contain unsolvable problem.

Likely your sister and mother have their own enmeshnent-based "alternate universe", where their bad behavior is unacknowledged and excused with similar sorts of rationalizations.  Don't try to accommodate them.