6 yr old jealous of other kids

So 7-9-14 (Wednesday) will make 1 year that we have had our kids. 6 & 9 now. The 6 year old girl doesn't remember a whole lot about her "real" mommy and the life they had with her. She was 3 when they were taken from their home. She has heard her aunt, who they were placed with before us, talk bad about their "real" mommy & daddy and all the bad stuff that happened. Everything has been going great, but this weeked, during our 4th of July cookout, I saw that she was sad and I asked her what was wrong. She told me that she was jealous of the other kids because they all had their real mommy & daddy's. I really didn't know what to tell her. I did tell her that we loved them just as much as any mommy and daddy love their kids and that God knew that their "real" mommy and daddy couldn't take care of them like they needed to. Does anyone have any other advice for me to tell her? I know she's still young and doesn't understand, but I also know that this will come up again at least until she's older and does understand. My oldest boy, her brother, he does remember the stuff and he never questions what happened. He just asks occasionally when they'll get to see their "real" mommy, who sees them whenever she takes a notion which is hardly ever. But we make clear to them that when she messages us for a visit, we always plan one.