I need advice! Foster to Adopt

I recently found out about a 2nd cousin who was adopted out of foster care 2 years ago with her 3 brothers. She was placed back into foster care and needs a home (they kept her brothers). She is 7 1/2 and does have some Anxiety, anger, and attachment issues. My husband and I want to try and foster to adopt her, but my dilemma is this: I have 2 boys already at home; one is the same age as her (2 weeks apart and they have played together in the past), the other is almost 3. I want to do what is best for my family, but my heart is also bleeding for this little girl who could possibly complete our family. I hear a lot about how you shouldn't foster children older than yours. Do you think my boys will be ok? Am I crazy for wanting to do this? Anyone have good experiences they can share? I've heard all the nightmares already.