Two issues here....
1) Middle aged family member who drinks, abuses drugs, is in financial poop, unemployed and on unemployment.
What does it matter what this person does? You cannot control this person.
2) The middle aged family member has a live in who has a child who reportedly is not doing well in school. You seem to think this person is in the relationship for money. Given the financial poop, unemployment and all, this makes no sense.
What does it matter what the live in does? You cannot control this person.
Kids flunk school for all sorts of reasons.
No need to speculate or add to the drama. If they hit the family up for $$, no is a complete sentence. That's easier than getting your underwear in a bunch about something you have no control over.