I Feel Left Out

It’s the first time I really haven’t been able to figure out a way to give her what she yearns for and needs.
She came home from school a bit quiet and sad and when probed about why, just kept saying “I don’t want to tell you, I don’t want to tell you”.
Eventually it tumbled out of her that she wants a sister.
I assumed it was because of Anna and Elsa who she loves with a passion but no.. it was more real than that. Several of her cousins and friends have new little siblings and the phrase that she kept coming back to was “I feel left out.”
I felt heartbroken for her.
How do you tell a child desperate for siblings that she actually has four of them but she can’t see and touch and play with them?
I tried to explain that if there were more children at home, she would get less time with me, and that we are lucky we get to play and snuggle so much together but it didn’t convince her and it was a wake up call for me.
There are some things I just can’t fix or sort out for her, even though I desperately want to.  I also need to work out a way to start talking more about her  absent siblings in a way that won’t make all this worse.
She has great friends and lots of cousins to play with and is generally a very happy little bunny; but it hurts that I can’t provide this one thing for her.