1)
Yo'el was not mentioned before!
1.
Radak: Even so, he is taught to teach the lineage of his sons.
2.
Malbim: He was one of the Nesi'im; Nesi'us began with him and continued until Be'erah, the last Nasi when Melech Ashur exiled Reuven in the days of Pekach ben Remalyah (6).
2)
Did each of them have only one son?
1.
Radak: Even if they had more, only the heads are mentioned.
