I've often heard about this type of thing happening and it's very normal when there are siblings involved, and especially a baby.
The best idea I've heard when it comes to dealing with it is to give each child some private "Mama Time"... they love it. Like plan some things to do with your eldest, just the two of you. The first thing that came to mind (and I just have one daughter, so I haven't needed to put any of this in practice!) is things like giving each other a manicure complete with nail polish. My niece loves that. Of course, it would depend on the age of your daughter... if she's ... well like a teenager, I'm sure she wouldn't appreciate that as much as a 7 year old might.
Cookie baking session with her favorite kind of cookie... just you and her in the kitchen? Museum or movie for the two of you? Window shopping at the mall? Since she's older than the others, maybe letting her stay up an extra little time at night to do something with just you... like reading together or a craft?
Those are just the first few ideas that came to me. I hope you'll be able to find something that will work for you!