My son and step daughter have been doing stuff

My step daughter told my husband she has a crush on her step brother and we later found out they had sex. So here is some detailsI dated my husband for just 1 year then we married. During that one year my son and his daughters didn't have much interaction besides a few meetings, he has 2 daughters but his oldest is 22 and lives on her own. So my son and his youngest daughter are 3 months apart in age but both are 15. When my husband and I married we considered moving into his house but I liked my neighborhood better so he and his daughter moved into my house, we turned the guess bedroom into her room.Okay, now we thought they were getting along pretty well and guess we were right. She said they used condoms that my son bought and they planned it for 3 days. She said that her and my son kissed the first night her and my husband moved in. They have been doing this for 5 months. My husband thinks we absolutely have to do something but I just don't know what. When I was a teen and I wanted to do something bad enough, I did. Combined that with the fact that they live under the same roof I just don't know. Advice? via /r/Parenting https://ift.tt/2QNqYeO