I think what Eliza is saying is that if it's a relationship (relative), you can tell by the 'a' or 'o.' I would guess that this sentence could be referring to a friend, too (isn't that 'a' class?) - just as in English, the sentence leaves you with not all of the information. CORRECTION: Reading further in comments, I see that the dictionary def is 'The youngest member of a family' so that means it can't be hoa. (Decided to leave my comment in in case someone else jumps to the same conclusion I did.)
The translation given here was my first choice until in my search for the noun described by the adjective "youngest" and couldn't find one. So I moved the noun "boy" to the end of the sentence to make sense in English, as that is occasionally seen on DL. I would love to see some explanation given for some of the more confusing lessons
[LA] 1 n The youngest member of a family; ka hanau muli loa; the youngest born of several children. The younger of two children of the same sex; an endearing appellation.
(duolingo) pōkiʻi = youngest sibling of the same sex, youngest child, youngest.
This favorite child is the youngest. "'O kēia keiki punahele ka muli loa."