The phrase "sit in for" can be replaced with several other synonymous expressions. One such phrase is "substitute for," which indicates that someone is stepping in for someone else temporarily. Another synonym is "fill in for," which has the same meaning as "substitute for." "Stand in for" is also a noun phrase that means to replace another person in their absence. Another expression that can replace "sit in for" is "act for," which suggests that someone is performing a role or function on behalf of someone else. Finally, "take over for" means to replace someone in their job or responsibilities.