I did not expect this post to blow up so quickly! Thank you all for your responses.
Armee: You know I spent all day wondering what people were going to say, low key hoping someone was going to deliver the shock I needed to break out of denial and by God, you've done it. (At least for now. These things come in cycles

) So I read up about emotional incest and it kind of falls in line with some of things I've been reading about enmeshment, codependency, parentification, and the like. I also feel like emotional parentification (taking care of your parent's feelings) and emotional incest (acting as your parent's life partner) can overlap. For example, as an adult, do you talk about work stress with your parent or your partner? Who takes care of you when you're sick? To M's credit I remember her hesitating because she didn't want to "burden me with grown up stuff", but it's not like she had anyone else (parent or partner) available. And I didn't often see her willing to talk about feelings or ask for help, so when she did, I seized the opportunity. Guess who has significant difficulty talking about feelings and asking for help
now?
Blueberry: Several things here. First, thank you for validating my reactions and that "once can be enough". My M certainly tries to be in tune with my sister, and sometimes she is, but she just doesn't have the resources to keep it up. Sometimes she remarks that she doesn't remember me being "like this" (read: loud and needy) when I was a toddler, and I wonder if she just had more energy back then, if I was naturally a quieter child, or if I had already at that age started learning how to be convenient.
Your early concepts of love in your FOO reminded me of how I used to think the polar opposite. I grew up "paycheck to paycheck" poor for the most part, and thus thought rich parents couldn't
really love their kids because they would just shower them with gifts instead of real attention. And they wouldn't even have any hardships to bond over! Makes me laugh to think about.
Papa Coco: I remember being less free than other kids too. I think the first time I went outside to play by myself, I was nine. We had been living in an apartment up to that point and it wasn't the seediest area (other kids were playing there) but it wasn't really a lazy suburb either. But even after we moved to a house there were rules -- suggestions, rather, that I wouldn't contradict for fear of being another thing stressing her out. Like texting her when I got home from school, staying on our block so that I couldn't be picked up off the side of the highway nearby, only crossing at crosswalks, etc. The irony was that I spent more time alone than the average kid (several hours every day after school from age 11 onward), so hypothetically I
could have broken some of those rules, but it was just unthinkable. If I did that, she'd have to
worry about me.
Master of my sea: I congratulate your courage in telling your story despite the guilt. I'm trying to do the same.
woodsgnome: I can't say I see what you see, regarding the resemblance in our relationships. No need to explain further though. I recognize that I'm fortunate the distance between me and my M "created itself"; I didn't have to escape. I'm not sure I would have even noticed anything was wrong and wanted to break that closeness, otherwise. And I can watch out for similar patterns in my adult relationships.
Blueberry again: No worries

Everyone deserves support and it comes up organically in conversation.