My brother-in-law lives with his parents. Will I be his ‘keeper’?

Dear Quentin,

I am looking for your advice about a tricky family situation. 

I have a brother in law, “Mark,” who is not financially competent and who still lives at home with his parents, despite being in his mid-40s. Mark has in the past lived on his own with a partner, but that relationship ended a number of years…