A complicated disorder to discuss on a chat forum.....but in a nutshell and you can Google for further information.
The prognosis for MND is basically3- 7 years from the onset of symptoms BUT there is a group who's symptoms start with problems in their hands or limbs which deteriorate slowly and have a long term prognosis in excess of 10+ years.
However, if it starts in the throat, then the disease follows a rapid course and it may well be a form of ALS (Amyotrophic Lateral Sclerosis) and it does sound as if your SIL falls into this group.
Sorry.