Rent Increase due to Tenant's Mom

[Ayodeji Kuponiyi]

A few months ago my tenant's mother expressed her interetst in moving into my unit (I have a duplex) when I informed her that I was looking to move out in a few months. Her lease expired and I agreed she could stay wth her son(my tenant) until I found a house. Now that I'm under contract, I  asked her if she's still interested and she said no. 

No problem, I asked her if she plans to stay downstairs or if she's looking for another place that fits her criteria. She hasn't responded to my text and this was an hour ago. My tenant pays $900 rent and if she stays I plan to increase it by $40. Am I asking too much? I pay the water bill for the duplex. Only her son is on the lease.

Any thoughts and advice are welcome.