-don't make her a second parent to her younger siblings. normal things like babysitting are okay, but don't make her a parent, let her be a sister
-don't compare her to her siblings.
-try not to make her feel like she has to stay strong for her younger siblings. as an eldest daughter, i felt like i could never let it show when i was struggling, or it would negatively impact my siblings, and that led to me not opening up or asking for help, internalizing my issues, and making everything worse.
-let her have her own personal space. (i'm not talking about her own room, because often it isn't possible to give siblings their own rooms). don't make her feel like she can't have alone time or can't have toys that are only hers. sharing is great, but don't make her share everything. too often i had toys that were for older kids, and my siblings were allowed to play with them, so they ended up broken. too often i would be playing with something/using something and my sibling would ask for it, so i had to give it to my sibling just because they were younger
-building off the last point: i understand wanting the eldest daughter to include the younger siblings when the hang out with friends, but don't make her do this all the time. don't make her sacrifice her time with friends because she also has to drag her sibling around. let her have time alone with her friends. i understand that to a younger sibling it can feel like being excluded, but your eldest daughter deserves time her kids her own age. (most of the time, as the kids get older, they will choose to include each other without being asked).
-don't dump your problems on her. this is a fine line to walk. talking about your issues can be fine most of the time, but confessing major issues (financial, serious mental health issues, large marital problems) can make your eldest daughter feel like she has to fix it, and that isn't her job.
-try not to complain about the "teenage years" while your daughter is a teen and her siblings aren't. even if it's a lighthearted joke, it can make her feel inferior or unloved compared to her siblings
-listen to her! if your daughter has a bad gut feeling about someone in your/her life, trust her. if she thinks one of her siblings is going through something in secret, listen to her.
-if she tries to point out something you've done that has hurt her or she thinks was handled poorly, please try not to take offense and listen to her. most of the time she isn't mad, she just is trying to fix patterns she's noticing
-try not to make your trauma her trauma. don't expose her to stories of your trauma too young. don't make her feel at fault for anything.
-make sure she knows that she doesn't need to have the same future you have! she doesn't have to be a mom if she doesn't want to: don't assume she will be a mother. don't beg for grandkids, don't bully her for not wanting children. her future is up to her, not you, and while you might want certain things from her, you're not entitled to them. she is her own person who deserves to make her own life choices
i might think of more later, but the most important thing to remember is that while these are great things to follow, mistakes are not the end of the world. parents are never perfect, and k can promise you that as long as you have good intentions and are willing to fix your mistakes, your relationship with your daughter will be just fine. if you accidentally slip up on these pointers, that's okay. as long as you want to be good for your daughter, the relationship will always heal.