John Gill Commentary 1 Kings 2:21

"And she said, Let Abishag the Shunammite be given to Adonijah thy brother to wife." — 1 Kings 2:21 (ASV)

And she said, let Abishag the Shunammite be given to Adonijah
your brother to wife .
] For so Adonijah was by his father's side; and Bathsheba makes use of the relation, the more to move upon him to grant the request.