Help with Tenant - Short Term visitor turned out long term

[Chait Borade]

Recently bought 4plex property, owner occupied and inherited tenant. There is a father son duo living in one of the units (2beds,1bath), who pay rent on time and are generally good tenants. Over the summer one of their family member started staying in the unit. Thought it was temporary. I found out he has been sleeping in the garage and basically moved in. I inquired and tenant wants the family member to stay with them until end of lease, they plan to move out at end of lease as they plan to purchase a house. Tenant claims the new member is "under disability" and should be considered as their dependent. Given everything going on with COVID, unemployment etc... I want to give them the benefit of doubt. *All individuals are above 30. I understand the 3rd person needs to be on the lease if I decide to let him stay. 

Questions for the BP community:

1. Should I allow a 3rd person in a Unit intended for 2 people. Given that the unit is occupied by 2 adult males I doubt anyone will be willing to share their room with another adult. I researched but I couldn't find any

2. Should I charge additional rent? I am not sure if this 3rd person has a job. 

3. What would you do in this situation

Your comments/advice is much appreciated.