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A "nonviolent communication" approach is: focus on what you are feeling and needing. And then think of some strategies to meet your needs.
Cary's advice focused on your parents - but I liked the way your letter focused on what YOU need - as a "moral being" - and I'd support your doing more feeling & strategising about that.
As an example of a possible strategy, if I were you I'd be wanting connection with my siblings (both of them) because (1) they are bearing the burden of your parents' aging, and (2) your siblings are going to be around for lots longer. You could offer your sibs support for the stress they are experiencing, or money to help with the parent-related expenses they face, while still observing your parents' no-go zone.
You and your family-of-choice are already role models for the rest of us "lesbian daughters". Thsnks for writing about this.