My heart to you! What a tough situation for all of you.
A book comes to mind. (As usual with me) Siblings Without Rivalry. Same people who brought me my favorite parenting, peopleling books...Faber and Mazlich. Those are also worth checking out...Liberated Parents, Liberated Children and How To Talk So Your Children Will Listen and How To Listen So Your Children Will Talk.
Based on the work of Haim Ginot. Also worth exploring. "Between Parent and Child" (for one)
What comes to mind from that source is to imagine you are a wife and your husband brings another wife home. That is in a way what your daughter is experiencing.
It wouldn't feel good to hear, "Well, you are just going to have to get along. Don't be selfish."
Instead, you would need reassurance, a bit extra for awhile, support for whatever effort, if any, you do put out there.
I'm not saying you aren't doing this, no idea. I remember a similar time with my toddler having a new brother. I had to put in some extra time on him. Show him that there is no less when it comes to mommy love. Acknowledge that it is a tough transition.
I would be doing special little things, letting her know you know this isn't easy and that you miss your just girl time too.
Then, slowly, ask her what can you do as a family to maybe help the brother feel welcome in this den of girls.
I remember having a talk with my oldest.."We have been so close, it must be hard for the baby...I wonder what we might do.." And my oldest went over and played with him.
He gave him a toy and said "Welcome to the family." I cried.
He grew into a big brother that protected his little brother on the bus and in Bible School. When either of them is out shopping with me alone, they just about always ask if they can get something for their brother. They even ask to use their own money so it is more special.
Not always a smooth road...there is often "Stuff"...but I find there is a lot less when I start with where is the child, not where do I want him to be.
Sorry to ramble on...my heart to all three of you!!! That has to be a hard transition.