My Stepson's Brother (Not With Us) Disclosed Mom Molested Him

We had suspected this for some time. My stepson came to live with us permanently at the end of January of 2013. There had been many issues and his mom had had an open case for about two years with CPS before all her children were removed. The children were adjudicated neglected and we went through the family courts for several months until full custody was officially granted to my husband. A forensic investigation is taking place, and we are pretty sure that this is not a false accusation. We are aware of some issues first hand and information from the GAL and family members that support this accusation. So, we are not part of the Family Court proceedings. We believe TPR is imminent. If TPR does occur, we will have to initiate separate proceedings for my stepson. How do you explain this to a child? He's currently seeing a counselor...We're preparing for the possibility of me adopting my stepson, but this is going to be hurtful no matter what. I'm also wondering if we should go ahead and initiate TPR due to the adjudication for neglect in light of this situation. --We believe he has been molested as well. We want to protect him.