Hi MsMystery. The story you shared sounds like your sister is high-maintenance. I can understand your frustration if this is a very frequent tendency of hers. I can't help but wonder what she expects from you or if anything you could offer, without totally being untrue to yourself, would even satisfy her. If you think there's something you could do/say that you're OK with, I'd just do it. There does come times when we have to just eat our pride to get stuff to pass. If she continues to dwell on the situation, I'd announce the "case closed". If you think there's never any satisfying her, then your choice could be to lose contact or just let some of her complaints roll off your back, as if your back was Teflon. I know that can be easier said than done, but it could be worth practicing. If it escalates her grievances, then she's got to know that it may be more "her problem" than yours, and that she might wish to talk to a therapist to help resolve them.

Another option you might consider is to ask a third party to provide some input into the situation. Are you overlooking something you may have did/said wrong, unknowingly? If so, then you may wish to process that with your therapist.

Do you actually truly value your relationship with your sister? Or are you more going through the motions? If the latter, why keep doing that? If the former, keep working at. We're not obligated to have relationships with people we clash with. There has to be some reciprocal benefit, even if small.