Othniel was probably Caleb's younger brother; the expression "son of Kenaz" being only an equivalent for the "Kenezite" Joshua 14:6.

And Othniel the son of Kenaz, the brother of Caleb, took it,.... The relation of Othniel to Caleb is a little intricate, and, as it is understood, occasions objections to the marriage of Caleb's daughter to him; it seems, at first sight, that he was Caleb's own brother, a younger brother, and so uncle to his daughter, and such marriages were forbidden, Leviticus 18:14. Jarchi thinks he was the brother of Caleb by his mother's side; Kimchi, both by father and mother's side; but not to observe, that besides the word "brother" sometimes signifies only a kinsman, or near relation, and not precisely a brother; it is not Othniel that is called the brother of Caleb, but Kenaz, who was the father of Othniel; so that Caleb was Othniel's uncle, and Achsah and Othniel were brothers' children, or first cousins, between whom marriage was allowed of:

and he gave him Achsah his daughter to wife; according to the tenor of his proclamation, and the promise he made.

Othniel took the town and received the promised prize. Othniel, according to Judges 3:9 the first judge of the Israelites after Joshua's death, is called כלב אחי קנז בּן, i.e., either "the son of Kenaz (and) brother of Caleb," or "the son of Kenaz the brother of Caleb." The second rendering is quite admissible (comp. 2 Samuel 13:3, 2 Samuel 13:32, with 1 Chronicles 2:13), but the former is the more usual; and for this the Masorites have decided, since they have separated achi Caleb from ben-Kenaz by a tiphchah. And this is the correct one, as "the son of Kenaz" is equivalent to "the Kenizzite" (Joshua 14:6). According to Judges 1:13 and Judges 3:9, Othniel was Caleb's younger brother. Caleb gave him his daughter for a wife, as marriage with a brother's daughter was not forbidden in the law (see my Bibl. Archol. ii. 107, note 14).

And Othniel the son of Kenaz, the brother of Caleb, took it: and he gave him Achsah his daughter to wife.

15:13-19 Achsah obtained some land by Caleb's free grant. He gave her a south land. Land indeed, but a south land, dry and apt to be parched. She obtained more, on her request, and he gave the upper and the nether springs. Those who understand it but of one field, watered both with the rain of heaven, and the springs that issued out of the earth, countenance the allusion commonly made to this, when we pray for spiritual and heavenly blessings which relate to our souls, as blessings of the upper springs, and those which relate to the body and the life that now is, as blessings of the nether springs. All the blessings, both of the upper and the nether springs, belong to the children of God. As related to Christ, they have them freely given of the Father, for the lot of their inheritance.