Do you know if she went out with him? If so, then that's really a no-brainer. To say nothing and ignore is basically giving permission.
If it were me, I'd say something like either this will stop (and you will need full transparency) or I will be filing or whatever. I and this family won't be disrespected like this. And then you have to be prepared to back it up. If her name is on the lease/mortgage/etc, you can't really ask her to leave, but you can say you aren't sleeping in the marital bed anymore. Others may know more, though.
Waiting until the new year for what? That's only a few weeks away. But the foot needs to be put down on the OM.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.
