From Longman Dictionary of Contemporary Englishsit in for somebody phrasal verbINSTEADto do a job, go to a meeting etc instead of the person who usually does it This is Alan James sitting in for Suzy Williams on the mid-morning show. → sit→ See Verb table
Examples from the Corpus
sit in for • Michael Morris will sit in for Betty Boothroyd ... the first woman Speaker, who was elected last week.• Daughter Catharine sat in for him while faculty and students wrangled through eighteen nights. 