What can I do about a job share that doesn't do anything?

[Eyeballls]

I went back to work in Jan and requested that my ft role be made pt. They agreed but decided on job share as it needed a full time person. Fine, no problem with that. So each of us is line manager to another member of staff; so a 'team' of four. The job is such that if one of our staff is off it falls to us to cover as well as do our own job, a fact which drove me the edge when I did the role full time but is not likely to change.

The situation now is that my member of staff has been signed off sick for stress for the next few weeks. So I have to do my job and hers as well as a big project I am doing but my job share isn't. My job share has her member of staff and no project but she isn't doing any of the work I leave for her to finish. This is all stuff that needs to be done asap and I do as much as I can but I already stay late and miss lunch to do the basics of both jobs.

My manager doesn't seem to know what she does with her time and is not the most assertive person in the world so won't really push her. I never see this person as we work 3/2 days and have no handover. I have told her stuff needs to be done asap but ultimately I am not her manager and there's not much else I can do. Every Monday morning I come in to the same piles of paper I left notes on and emails from people complaining that they were waiting for things that didn't happen.

Sorry this is a rant but I seem to be doing three people's jobs and I've had enough.