Foster with 4 year old daughter?

We've been trying for a while now.  With our dd, it wasn't this hard.  Fertility treatments, appointments an hour away once a week, and I'm 38.  So, I have been looking into foster care. I know NOTHING about it, but I'm interested.  I know we would give a child the most love and a good home, but I'm concerned about bringing a "sibling" in for my dd to get attached and possibly have the child sent back to their parents.  I think I could prepare, but I can't imagine that for her. Advice?