Is there a reason only "brother" is correct and not "sister"? Doesn't it depend on the speaker's gender?
I'm not convinced that "the son of my younger brother" is really different from "my younger brother's son." I will report.
Still not accepted 7/22/20. Reported -- again
Maybe for "my younger brother's son" it should be "ʻo keola ka koʻu kaikaina keiki kāne" instead of "ʻo keola ke keiki kāna a koʻu kaikaina" ?
Kaikaina can mean either younger sister OR younger brother is the notes below are accurate. This means my submission using younger brother should be accepted.