See The Scots Peerage, vol. 5, starting at p. 519 - there are multiple problems with this profile and he wasn't married to Isabel Wemyss (or at least his wife appears to be unknown). Nor are his parents known for sure and he this family weren't officially Lord Fraser of Lovat until a couple of generations later.
His son Hugh, did marry as his second wife, Isabella, daughter of Sir John Wemyss of Wemyss (see p. 522 of same source).
I notice back on the profile for one of Hugh's purported ancestors Sir Simon Fraser, that there is a comment from 2015 that there are problems, and really I think at some point this whole family line needs a complete overhaul.