The only fic I can think of with Sarek acting fatherly, mainly towards Spock and to a lesser degree towards Jim, is Keira Marco's series called Tangled Destinies (http://keiramarcos.com/star-trek/tangled-destinies-ks/).
It's AU, with Spock and Jim meeting as teenagers, and Jim being part Betazoid. An excellent read even if it's not exactly what you're looking for.