Q: I am in grad school and living in my parents' house. My issue is that my sister, who is a few years older and who also still lives at home, acts as my third parent. She does not have much of a social life and her love life is nonexistent, and she acts as if normal things I do, such as spending the night at my boyfriend's apartment or going out drinking with friends, is disrespectful and rude to our parents. To add to it, she is my mother's favorite child, and can do no wrong. What can I do?!
A: You can step back far enough to see that you're the one with all the advantages here.
Yes, you're stuck in your crowded childhood soil when you're desperate to replant yourself elsewhere. But even that disadvantage reveals an advantage: With the exception of your finances, you're ready in every way to be independent. I suspect Mom's favorite can't say that.
So instead of re-fighting old family battles, even if just in your mind, please see the surprise benefits you've accrued. Your sister is in your business because she's too enmeshed with family to have business of her own; your mother's favoritism was a disservice to both of her children, but arguably it was worse for your sister. At least feeling less welcome at home stands as motivation for you to get out.
That won't help you much, of course, if you don't actually get out. Just being sucked in by your sister's criticism says you're at risk of becoming ensnared by family in your own way.
While you're working to cut the financial strings, you also need to work on the emotional ones. You owe your parents basic courtesy, but with those boxes checked you can and must disengage from sibling supervision: "(Sister's name), I hear your concern. If Mom and Dad share it, then they can tell me directly."
If your parents do object to your choices, then openly and honestly discuss ways to be yourself without imposing on them. That's the deceptively simple foundation of grown family harmony: All adults speak for themselves.