It had the atmosphere of a soap opera plotline IMHO. Young women rarely are terminally ill and the likelihood of that coinciding with a bit on the side opportunity seems to stretch credulity. Not saying that OP is spinning a yarn, nor that the woman is either, just that it's odd...
Shame the thread is closed - was just about to post that the OP should definitely leave his wife & 4 kids for the terminally ill other woman as nothing could possibly go wrong for him in that secenario