Tenants moved in adult child. Do I need to evict?

[Roberta Workman]

Just found out tenants moved in their son (over 18) 2 months ago. Lease says no more than the original 2 tenants.  55+ community with covenants saying underage visitors no more than 14 days. I bluffed and told them he needs to move out, but do I need to formally evict just the son? With the current rulings I assume I can't evict him until this is over. I know, I should have caught it sooner, but they're the kind of tenants you only hear from when something breaks and unfortunately for them (and me) the septic backed up requiring my attention.  Tank was pumped 3 years ago before they moved in and I'm sure having an extra person on it didn't help.  I have water damage to the laminate flooring in the hallway.  Do I take it out of their security deposit?  They're on a month-to-month at present, slightly below market rent.  Perhaps I should just suggest they might want to move to a more suitable dwelling.  They're crammed up in less than 1000 sf.  What to do?