I accept the possibility of a genetic link, but still do not think that environmental contributions should be completely excluded.
On of the suggested links is the inclusion of adjuvants such as squalene in vaccines. There is some variation from batch to batch and even dose to dose within a batch. As MS can take many years to become recognized, it is possible that receiving a vaccination many years ago could have been the innoculous that started the issue.
It also could have been an illness common to the family members that occurred many years ago.