Hi,
WaveRide is spot on with her recommendation.
Yes, if your mother in law cannot look after her own nutrition and meds someone needs to do it for her. If she won't or someone else can't yes she will die, and likely sooner than later.
She needs to see dr asap regarding her leg.
Not what you wanted to hear, I'm sure, but it's the truth.
Best wishes
Take your MIL to a doctor ASAP to treat "Her entire foot and calf now have a black undertone with black veins(or atleast that what it looks like) and are swollen." This sounds like a serious infection such as gangrene or the onset of diabetes neuropathy. Both, if left untreated, leads to amputation.
Also, your MIL needs someone to look over her nutrition so she doesn't eat junk foods and to monitor her medication. The latter making sure it is brought to her and taken as prescribed. You cannot depend on her to act alone as she has neglected and ignored this far too long. It is time for someone, such as your wife, to do it for her. Bottom line take her to a doctor today.