... Keep expectations, of her, age appropriate.
Otherwise, frustration will occur, for her and you.
Ditto the others here.
Just because a child is now an 'oldest' sibling, it does not mean, that the child suddenly grows up and matures. They are still... their same age.
And a young child.
The 'expectations" a parent has, upon the Eldest sibling... also can cause, the child to act out or not, or sabotage them or not. IF the parental expectations, upon the older child, is not age appropriate, then the child will never be able to meet those parental expectations. Thus, the child will get frustrated/resentful/unhappy/and stressed. ie: turning into a 'brat' too, at times.
For me, my daughter was about that age when I had my 2nd child. I ALWAYS, talked with my daughter, and explained baby development to her, in ways she could understand. So that, she understood, how her baby brother was/is/and what he was able or not able to do. That a baby wakes/cries/feeds from Mommy.
Because I explained things to her, about her baby brother, it helped her to gain understanding... and to not have to wonder aimlessly, about what is going on or why I have to tend to baby.
Each month, and at each phase of her baby brother, biologically and emotionally, I explained to my daughter, about what was going on.
Unless a child is explained to, they will not automatically 'know' how it is. Nor their place in it all.
I also, with my daughter, talked with her a ton, explained emotions and her feelings, I "allowed" her to have emotions and to tell me anything... and I validated her. I told her "Mommy knows, this is not easy for you either.... but WE ARE A TEAM.... and I am always here for you." I also told her to "Remind Mommy if you have to, when you need me.... because Mommy is so busy and tired. But I don't know everything either... but you are important to me. Please remind me too, of your feelings..."
Thus, I made her a part... of me and made sure she KNEW, that we were a TEAM.
I told her 'You are always, my first baby.... I love you...." even when I was nursing my 2nd child.
All of these things helps.
The Eldest child, NEEDS to be validated, and commiserated with, too.
It is their home, their family, and their baby too.
Their life is no longer as being an only child.
They have a ton, to adjust to.