Genesis 10:21

אבי כל בני עבר THE FATHER OF ALL THE CHILDREN OF EBER — (עבר means the “side” or “ other side” of a river) — The father of all the children of עבר הנהר, the other side of the River (Euphrates), was Shem. אחי יפת הגדול [SHEM …] THE BROTHER OF JAPHETH, THE ELDER — (The Hebrew is ambiguous: “the elder” may refer either to Shem or to Japheth). One cannot from here determine whether Japheth was the elder or Shem. Since, however, it states (10:10) “Shem was a hundred years old … two years after the flood” you must admit that Japheth was the elder. Because Noah was 500 years old when he first had children (5:32), and the Flood happened in the six hundredth year of his life (7:11), consequently the eldest of his sons was then one hundred years old, whereas Shem reached his hundredth year only two years after the Flood (and therefore Japheth was the elder of the two) (Genesis Rabbah 37:7). אחי יפת THE BROTHER OF JAPHETH — It does not state “brother of Ham”, because those two honoured their father whereas Ham put him to shame.

וּלְשֵׁ֥ם יֻלַּ֖ד גַּם־ה֑וּא אֲבִי֙ כׇּל־בְּנֵי־עֵ֔בֶר אֲחִ֖י יֶ֥פֶת הַגָּדֽוֹל׃

To Shem also, the father of all the children of ῾Ever, the brother of Yefet the elder, to him were children born.