Who do I screen exactly ?

[Som Jafari]

Hi I'm buying a duplex to house hack. There is family paying month to month in the other unit and I'd rather have tenants in place so I don't have to deal with the hassle and expenses of finding new tenants. They consist of a husband and wife and their young daughter and the wife's parents (five people total in 2bd/1b unit). The lease contract is with the wife's brother who has left and is living elsewhere. I told the wife (haven't met the husband and wife said he doesn't speak English) that would not work and I would have to make a new contract with the people who are actually living in the unit. 

They've been there for four years and taken good care of the place. They are already paying the rent I want. 

My question is do I have to run background and credit check on all four adults or just the person paying rent? Anyone been in a similar situation? What should I look out for?