Is she daughter of Gilbert Kennedy, John Kennedy, or we don’t know?
http://venitap.com/Genealogy/WebCards/ps31/ps31_181.html
The son of the latter, Sir John de Forbes, Lord of Forbes, called "Sir John of the black lip" was Justiciary and Coroner of Aberdeenshire. He married Elizabeth Kennedy, daughter of Sir Gilbert Kennedy of Dunure and together they had four sons. From the three younger sons sprang several cadet lines.
William was the progenitor of the Pitsligo line,
John the ancestor of Tolquhon line while the
houses of Skellater and inverernan were founded by Alistair of Brux.
Sir John died in 1406.
Source: Wikipedia: http://en.wikipedia.org/wiki/Clan_Forbes