Can a parent be liable for knowingly allowing underage kids to have sex in their house? Both are under the age of consent.

We have been dealing with a very aggressive girlfriend for the last year and have on four occasions had to ask her to leave our house because of her inappropriate behavior (over the top PDA when younger siblings are around). We have also banned him from going to her house at any time because the same concerns. We recently found out that our concerns were very justified when we were told that her mother knowing allowed them to have sex in her house while she was home on two occasions before we stopped letting him go there. I believe in terms of consent (he is 16, she is 15) they are both committing (and a victim) of a crime because both of them are under 17. Did she commit a crime similar to allowing underage drinking by knowing allowing this in her house and even encouraging it.