Tenant's family coming to stay... Can I raise the rent?

[Gregg Jossie]

I'm househacking my first duplex right now and my tenant informed me today that his daughter, son-in-law, and their five kids were just evicted and are coming to stay for a while. He doesn't want them staying long and neither do I. The lease only allows for guests up to 30 days. Anything longer than that needs my permission. This family has stayed here before and hasn't caused any issues, but I want to know if it's okay to raise the rent if they end up staying longer than the 30 days. My tenant is 7 months in on his 2 year lease. He's paying at least $200 below market anyway, but it's a tenant and lease that I inherited when I bought the place. I'm thinking if the family stays longer than 30 days, raise rent by $200 per month that they are there. What problems might I encounter with this plan?