sit up with
sit up with (one)
To stay awake past the usual time to go to sleep in order to care for one or keep one company. My daughter kept vomiting, so I had to sit up with 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.