I am not sure of this wife, or at least her ancestry, but a dozen or so internet source have her as husband of Thomas Erskine, some even with a second husband of Patrick Hepburn, 1st Earl of Bothwell (this latter marriage in 1 Feb 1480/1 is given in Burke's Peerage with no mention of a prior marriage). The Janet (daughter of a 1459 marriage between James Douglas, 1st Earl of Morton & Joan Stewart, deaf-mute daughter of James I) that married Patrick is far too young to be the one marrying Thomas Erskine. I could be wrong because some people put that as the case.
Father: James 2nd Lord DOUGLAS , of Dalkeith b: ABT 1407 in Dalkeith, Midlothian, Scotland
Mother: Elizabeth GIFFORD b: ABT 1409 in Sheriffhall, Newton Parish, Edinburghshire, Scotland
Marriage 1 Thomas 2nd Baron ERSKINE , 13th Earl of Mar b: BEF 1418 in Erskine, Renfrewshire, Scotland