sit for someone—
1. serve as a babysitter for a child:
- It was easier for them to leave Bernard, who stays up forever reading anyway, to sit for his brother.
2. work as a babysitter in smb.’s employ:
- I sit for Mrs. Franklin every now and then.
3. serve as a model for an artist:
- She had promised to sit for him just for a lightning sketch.
Note: The expression does not correlate in meaning with the phrase sit up for someone—not go to bed in expectation of the arrival of a person:
- There are few things more worrying than sitting up for somebody.
sit in for someone—attend a session, sitting, etc. as a substitute for a person:
- I am not a regular member of this committee. I am sitting in for Larry Smith.