TopsyandTim Whatever the reason, looking after your grandchildren is clearly more than you can cope with at the moment.
Speak to your SiL and tell him straightforwardly that at the moment you are rundown, both physically and mentally, and will have to stop caring for the children for the time being, better not be more specific. Do not blame the children.
Try if you possibly can, to see a doctor for a physical and mental checkover. If your GP is unavailable, contact a private hospital. You can get a 30 minute appointment with a private GP for about £100.
But what you need to do before anything is stop your childcare duties, you are at the end of your tether and need a long rest.