Doing shift work wreaks havoc on your body. Your immune and endocrine systems suffer due to sleep deprivation, and not having a regular sleep schedule like everyone else. Combine that with the stress you've been experiencing from your husband and the twins, and it's no wonder your white cell count is elevated. It sounds almost like you're starting to experience a burn-out (maybe chronic fatigue syndrome or fibromyalgia? try to get those two conditions ruled out).
Are you in a position where you can get some paid time off from your job (vacation or sick leave for a few weeks maybe)? It sounds like you need some serious R&R. Would you be able to transfer to another position where you wouldn't have to work as many shifts? Or maybe get into a job-share position? I don't know exactly how it works where you are, but my doctor wrote a sick note for me not too long ago because I was having foot pain - my manager accommodated me by giving me a modified work arrangement. We have some RNs who cannot - for health reasons - work night shift (a couple of diabetics and another with thyroid problems) because their endocrine conditions are exacerbated by not having a regular schedule. The hospital is required to accommodate them. Check with your employer's Occupational Health service and your Union to see what you need to do to protect your job. Maybe you could transfer to another department where you wouldn't have to work night shifts or weekends (like if they have surgical day care clinics, etc).
It sounds like you really enjoy your career as a nurse - it's not something to give up lightly. How would it affect your pension? How much longer do you have before you can retire? Do you really think you'd be able to manage on a disability pension income?
Those are all things that I'd consider before I reached a decision like the one you're facing.
You need to take good care of yourself - it affects your work performance if you don't!
Hugs, Scully