Issues with Birth Mom - Need Advice

Hi. So we adopted two boys three years ago. Since then, it's been not so great. Basically, the older one went off the rails -- felonies, hurting others, almost daily drugs and drinking -- and went to detention. This was almost a year ago. The younger one was crushed because they had NEVER been apart and he, of course, missed his older brother horribly. At this point, he wanted to reach out to his birth mom. They'd always had some contact since before and after the adoption and it made sense that he would be looking for someone from the family since his brother was gone. Also, bio mom had been doing much better and we had always promised contact as long as it was not bad for the kids. So they started actually seeing each other instead of just talking over the phone and texting and such. It's been... okay. However, recently, I feel like bio mom is starting to try and undermine us. He has told us that she has told him not to tell us things, which is really upsetting. She is also carrying messages from the older boy to him (this is an issue because, after the older one when to detention for a while, the younger one started telling us that the older one hit him every day. Honestly, the younger one has done fantastic -- truthful, picking better friends, doing better in school -- and decided he didn't WANT any contact with his older brother right now. And I told everyone that we were letting the younger one decide WHEN he did want contact with him again because he was going to have control of the situation) and I can tell this is causing conflicted feelings in the younger one. I'm starting to feel like things are getting out of control and I'm not sure how to deal with it. I don't want to cause any more pain for my kid but I also don't want her being manipulative and causing more stress for him, either. I feel like she's being very dishonest with me, like if this is what I know about, what else is going on? Thanks in advance for the help.