sit up with
sit up with (one)
To stay awake past one's normal bedtime in order to care for someone. My daughter kept vomiting, so I had to sit up her through the night.
sit up with someone
to stay with someone through the night, especially with a sick or troubled person or with someone who is waiting for something. I had to sit up with my younger sister when she was ill. I sat up with Bill while he waited for an overseas telephone call.