Legal Question in Landlord & Tenant Law in Missouri

If your lease agreement says that rent is determined by the number of occupants, you can probably increase the rent. If your lease agreement limits the number of occupants, you may be able to declare the sister moving in as a breach, and thereby induce the renters to pay more. but, if the lease is silent as to co-tenants, and the unit does not violate fire or other codes, you may be powerless to raise the rent.

Good luck