don't know what to do

My stepson (12) mom called us today to inform us that his brother (5) had made accusations of my stepson touching him innapropriately gave very specific details and when and where I guess this all came out at his brothers daycare last Friday. He was with his mom this weekend and she has not left him alone since hearing it. but wanted to inform us because we also have a daughter that is 5... while us (his dad and I) and his mom don't see this to be true.. it is also concerning because if it really did happen then that is terrible. His mom is going to contact resources to get to the bottom of it.. it especially scares me because we are moving into a house we bought and our daughter and my stepson have their bedrooms in the basement because of a new baby due in 6 weeks needing to be upstairs with us.. I don't know what to do or what to think I don't think he would do these things his brother said but he has been having behaviors since a nasty court battle over the last 2 years.. the only 2 people I said anything to said just to keep our daughter upstairs with the baby ok yeah that's fine I'll do whatever needs to be done but it seriously sucks we are buying this house so everyone has their own space ect... and my husband was like if that really did happen he will end up living with us.. um ok but I'm worried about our kids to being subjected to that as well..