William's father married "first, the Lady Isabel Montgomerie, daughter of Hugh, first Earl of Eglinton, and secondly, Christian, daughter of Ninian, Lord Ross, of Hawkhead, a papal dispensation for which match, as within the prohibited degree, was obtained from the see of Glasgow, in 1538. The Laird of Caldwell had (with younger children) two sons,
"I. John (Sir), his heir.
"II. William, who received the lands, and founded the house of Glanderstoun, long a distinct Renfrewshire family, of the first respectability."
Burke's Commoners, I, p 455. So we don't know who their mother was until we find birth dates: but GenCircles has ideas, and the Geni date for William points clearly to Isabel.