Not my fault but I feel bad. Did I handle this right?

I apologize for the length, we finalized on our kids 5 and 6 two months ago. They have two younger siblings that were not part of their case but were living in the same Foster home with them. Bio Mom did not work her plan for our kids so her rights were terminated on them first. They had been in CPS care before and were RU'd originally. She wasn't working the plan for the younger two until the very last minute and was able to get it together and got the two younger siblings back. Our kids were moved to our home before their siblings went to Bio Mom. So we took our kids to visit with their siblings at the former FP's house before they moved. We also wanted to support their relationship with their former FP's because of the bond they have. We visit once every two months now. We gave them a picture of them as a memento several months ago. The last time we visited we invited them to come over for dinner at our home. Then they mentioned that Bio Mom had been visiting them as well so they could see the siblings. I was kind of shocked as they have other Foster placements and wasn't sure that was allowed for a Bio to visit a former FP especially after having their rights terminated on two of their kids. I didn't say anything and put it in the back of my mind. Then I come to find out that Bio Mom has the picture I gave the former FP's because somebody told me she posted it on FB! They are the only people we gave that particular picture to so I know they either gave it to her or they let her take a picture of it with her phone. They keep all their FK's pictures in an album, which I have seen, so there is no way Bio Mom just took it. I'm upset and worried since they now have our address because of our dinner invitation. I'm still in shock! I know I should have talked to the former FP's and tell them that we know they gave her the picture but I didn't know how to do it. They are nice people and I think they just felt bad for her. We have the same caseworker as the former FP's so I called him and let him know what happened and he said he would talk to them and take care of it. He was upset and very disappointed as well. Given their tenure as FP's this should never have happened. I struggle everyday with them not having contact with their siblings. I'm angry at the court for breaking them up at the last minute. Our kids are still too young to understand why they were placed in Foster Care and every time I try to explain it in a way they would understand they get confused. So I know, now is not the right time for contact. They don't even remember what their Bio Parents look like. We are more than willing to initiate contact when they so choose. I will be honest with my kids about what I know and will support them if they want to make contact. But I feel betrayed and I feel like cutting contact with the former FP's. Did I do the right thing telling the caseworker? I feel bad about keeping our kids from the former FP's but I don't know if I can trust them. I feel so dumb for giving them our information.