NC for this, but regular.
I'll try to keep this as brief as possible, but don't want to dripfeed so a bit of background is needed.
Sister is childless, recently bought her first house with her longterm partner, having lived at home to that point. She has mild anxiety and depression, but both she and her partner hold down well-paid graduate jobs. Despite owning her own home (which is beautiful) for 2 years, she still lives with my parents at weekends, including taking her washing there for them to do. She also spends all her annual leave there (she has literally never had a holiday elsewhere). She is 37.
Recently, due to a scheduling conflict, my parents pulled out of an arrangement to see Dsis and her partner for a week of annual leave. They didn't communicate this well to Dsis, and left it to the last minute to tell her, which was understandably upsetting for her.
However, Dsis is now refusing to speak to, or contact, our parents and says she feels that they have shown her "disrespect". (While I feel that they could have given her more notice, I disagree that they intended to show a lack of respect by it). She is accusing them of deliberately depriving her of any possibility of having a nice week off, because she can't possibly take time off in her own home with her partner and still enjoy it. Therefore, they have "ruined" her holiday and she can't forgive them for depriving her of "precious downtime".
What concerns me most is that she takes the position, and I quote, that our parents' refusal to have her over for the week means that they are "in dictating mode and there is very little that is less helpful that people telling you they know what you need better than you yourself do". In other words, she TOLD my parents she needed a week off with them, and they dared to suggest they had other things to do and that she really should get more used to spending annual leave with her partner alone. My sister thinks this is absolutely outrageous and that they owe her a nice time on her week off. There is no room her for my parents to have their own independent desires.
Important bit of context: my position to date has been to stand clear of this relationship and to let them all get on with it. I have, however, become quite concerned about things since my DM has significant MH issues and there have been hints that these are being (unwittingly or deliberately) exploited. In particular, my mother finds it very hard to say "no" (due to an abusive childhood) by herself, so it's incredibly difficult for her to set boundaries. What happens repeatedly is that my sister insists that there is a medical reason for her to get what she wants: she must have their company on a certain date because of her anxiety, they must do her washing because of her depression etc. Now it has become that my mother is "damaging her" by denying her the opportunity to have the annual leave she wants. My Mum has said explicitly she doesn't want to do these things like extra washing (she and my Dad are in their late 60s and both are cancer survivors), but she feels guilted into them.
I just don't know what to do. Part of me thinks I should continue to stay out of this as a toxic situation that is between them. Another part desperately wants to support and protect my parents, because I feel this is now reaching a point of entitlement/potential exploitation where it is becoming almost a mild form of abuse. WWYD?