Automatic rate increase for unknown tenant

[Johnny Pedroza]

I have given special consideration to a tenant based on her financial circumstances. Her contract is set to renew December 2018. How can I place a provision in her contract that stipulates if an adult moves into the unit, not listed on the rental agreement, I will automatically increase her rent? Side note; she is paying 1/2 the market rate to rent the unit. I want to try to deter the off chance of her moving another adult into the unit. Thanks