John Gill Commentary 1 Kings 11:19

"And Hadad found great favor in the sight of Pharaoh, so that he gave him to wife the sister of his own wife, the sister of Tahpenes the queen." — 1 Kings 11:19 (ASV)

And Hadad found great favour in the sight of Pharaoh
Perhaps for his comely personage, princely qualities, and good behaviour, as he grew up:

so that he gave him to wife the sister of his own wife, the sister of
Tahpenes the queen ;
it seems the kings of Egypt used to marry their favourites to great personages; see (Genesis 41:45) .