Applicant with unrelated minor - problems?

[Mila F.]

I never came across this. I have a qualified applicant for one of my properties who wants to move in with his minor (17-yr old) "stepson". The boy is from Latin America on a tourist visa here. I'm told the boy is the applicant's fiancee's son, so, at this point, they are not related. The fiancee is still in Latin America, dealing with documents so she can get here and marry the guy. They expect the process to take about a year. The applicant said because they are planning to file a fiancee visa for her, he (the applicant) can be the boy's legal guardian for the time being . I have no clue if this is how things work, I just wonder, as prospective Landlord, am I required to check if this is a legitimate guardianship and not something fishy to be happening at my property? Should I be requesting proof of guardianship, the boy's visa info, anything else? Or none of my business?
Other than this I don't see any red flags with the applicant.