going along with what YesOrNo said (and I repeat his brochos to both of you) I think communication is key. I don’t know what kind of communication you have with your children but it would be beneficial to talk to your daughter about her being your flesh and not at risk of losing her “place”. In words she can understand – let her know that you want her to partner with you in making this other girl feel welcome. Let her know that it is really hard to share the people you love but that no matter how much you give to this “newcomer” it will take nothing away from your love for her. And let her know that you understand it sometimes feels hard or she may feel jealous so she can always come to you for an extra special hug. But let her know that when she is feeling like acting out it may be because she is feeling bad inside and she should come to you instead of acting out because you want to hear about how she feels.
These may sound “lofty” for a child that age, but truly they are feelings and thoughts she may surely have and she will get it. It can give her the assurance she needs and the safety net (knowing she is allowed to feel anxious and ask for an extra hug). These are conversations I have had with children that age on their level, and you would be surprised how much it helps them to know that they are understood, and are not “bad” for wanting their “life” back.
Children understand so much more than we give them credit for and so many of their difficult behaviors or anxieties come from things we never would think of discussing with them. It is great for them to learn to be in touch with themselves as well as connecting with an adult for support.