Right now, it is more important that YOU recover fully before going back to work.
Much as we'd like to think we are totally indispensable at work and that 'no-one can replace me' - I used to think the same! However, looking at it logically - with you taking 4 weeks holiday every year is proof enough that they can definitely 'get along' without you, without you having the guilts. ;)
As @primek says - there are a raft of options (including temps) to get them thru tough times. Who knows - you may head in a different direction yourself, further down the track.
You were obviously very loyal to your employer when she had her stroke and they should appreciate you even more for that - I hope they are as loyal to you now, in your own time of need.
Take care xx