After a review of incest laws and "void or fictitious marriages" by states, over half of the states in the US indicate "step" as a prohibited relationship for sexual conduct. Most of the laws do not specify that the law only applies to minors but age 16 and above which would include this situation. That considered... the MC has every right to worry about legal ramifications and the fact that the girls are over 18 is irrelevant. That aside, as long as they are all consenting adults and keep the activities and relationships in the house, even if the ex-wife gets suspicious and causes an investigation, if there isn't any proof and none of the girls admit, they won't get into trouble. However, I would immediately delete all inappropriate messages between all of them on their phones such as the pictures and direct statements as they could be used as evidence if anyone got a subpoena for the records.