sit up with someone—keep a person company remaining late out of bed:
- Charlotte sat up with her late at night and the two of them watched the snow fall together.
sit with someone—
1. stay with smb. who is in need of care:
- Brigi sat with her boys in the hospital when they were sick, did their speech therapy with them, helped them with homework.
2. (of things) suit a person; be accepted by smb.:
- How will it sit with the bishops if Charles chooses to marry a woman with whom he’s been living in sin.