Indeed. I really think there are a wealth of story options and doors that can be explored by having her be Sarek's ward, and him be tasked to do so because of his ambassador status. As Sarek would say, "Taking her in was...logical."Not really - again:
- Burnam is a child whose parents (who are in Starfleet) are killed on Vulcan territory.
- Sarak IS the Vulcan Ambassador to Earth AND is married to a Human
^^^^
IDK - but if there was a Human child on Vulcan needing surrogate parents, Sarek seems the loigcal choice here.