Our expert says:
The comment you got is right - you could just replace the chair with the ball, that makes a difference already.
For other exercises, this is a pretty good resource: http://www.mayoclinic.com/health/core-strength/SM00046
Hope that helps. And also, I know it is difficult, but three jobs and no time for yourself is probably not that sustainable, not only for fitness but for general well-being, so hopefully your routine can be changed soon, giving you time to exercise.
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