It appears from internet searches that Serena, née BAYLIS(S), is the wife of Alexander Teagle KEITH, rather than his sister.
It also appears that the late Hon Alastair James Keith's widow is his second wife, (Anne) Wing, née SOMMERS (born 1958, and a descendant of King William I - see the genealogy at
http://www.william1.co.uk/t39.htm). She was previously married, on 16 January 1988, to Robert Orris BLAKE Jr (born 1957) - see the announcement of their engagement in the
New York Times at
https://www.nytimes.com/1987/10/25/style/wing-sommers-to-wed-r-o-blake-jr.html. See
https://en.wikipedia.org/wiki/Robert_O._Blake_Jr. for information about Robert Orris BLAKE Jr; although the article does not mention his first marriage, it does provide a link to the wikipedia article on his maternal grandfather, (Edwin) Sheldon Whitehouse (5 II 1883 - 5 VIII 1965), and some of his connections, including his sister's marriage to the second son of the 9th Earl of Coventry.