Is Coriantumr a descendant of Jared or of the Brother of Jared?
It seems likely.
In Ether 11:17 we read:
17 And it came to pass that there arose another mighty man; and he was a descendant of the brother of Jared.
This is the last we hear about who is king until we are told that Coriantumr is king and he is the last king of the Jaredites.
However there is a gap in the recorded line of the kings of about a generation at that time so we can’t really know.
Still to me I would have to say I think it very likely that he was.