Also if you are someone who sees people say or joke that "eldest daughter is its own gender," and think that's ridiculous:
Gender is culturally constituted.
(This is an incredibly basic tenet of both feminist theory and queer theory, btw.)
So, this was not a thing in the culture you were raised in, great.
But in a lot of cultures, eldest daughter IS its own specific gendered positionality.
Listen: One side of my family is from a conservative, religious immigrant community, and a very Catholic part Eastern Europe before that.
I've talked to my mom about her childhood. And you know what she has said to me about a lot of the girls she grew up with?
"Some of those girls had no freedom at all."
There are households where Eldest Daughter = Mandatory 2nd Mom, from an age as young as 5 or 7.
There are households where the boys can do whatever they want and go wherever they want, while the daughters are never allowed to go anywhere except school and church, and the oldest daughter is responsible for doing all the chores - while the boys have to do none - and taking care of every single younger sibling.
There are households where the eldest daughter's job is to protect the younger siblings interpersonally and from violence - including, painfully often, violence within the house.
There are households where the eldest daughter's job is to be okay, and help with or fully manage care work for younger siblings, until every single other person in the house has been seen to.
There are families where no one gives a shit about any daughter that isn't the first, and grandparents will legitimately ignore every other daughter.
And there are a lot of women who could only get an education because their parents had no sons.
Yes, sons and daughters younger than the eldest absolutely can and do get parentified as well, especially in times of strain. And that's fucked up and needs to stop. But in a lot of cultures, the eldest daughter specifically is expected to do far, far more parenting and work and housework than the actual fucking husband / parents. And this is by expectation and by default.
If you haven't seen this shit, great, I'm so glad for you that you haven't had to see or deal with this shit.
But that doesn't mean it isn't fucking real.