Those angles have also been covered, up to a point, but thanks for thinking of them all the same. The problem again is that the MS struck in adulthood whilst they were living in separate accommodation, so it's a bit of a stretch to believe, for example, that there was something toxic in all 3 (to date), potentially 4 houses that includes one well-maintained and clean flat.
Another thought occurs though that must also be considered. One or two posters on this thread have rejected the idea of a genetic link. I'm slightly less sanguine about that. There are at least 5 children involved here that I know of (ie born to the sufferers or a sibling), and whilst I don't want to be pessimistic about their life chances, the possibility that they also might eventually succumb to the same diagnosis has to be considered.
If, God forbid, this illness found a foothold in the next generation, then I would argue that the genetic link cannot be excluded as a possible explanation.