My late brother contracted Kennedy’s but I did not. My mother of course was the carrier.
I live in the Canadian province of Nova Scotia, population 1,000,000.
My brother’s specialist doctor told him that in the province, he knew of only one other case. This person was from the small hamlet of Nictaux Falls.
My mother’s father, my grandfather, was born in Nictaux Falls!
Is it possible that the gene was always in him and he passed it on to my mother? He did not have Kennedy’s and passed away at age 67 in 1943 from heart related problems. My grandmother was born in another area of Nova Scotia and died youngish at age 57. Her mother lived to be 88.
Is there any connection do you think between my grandfather and the village of his berth?

Here is a paper that suggests the KD mutation is several times more common than estimates based on rates of diagnosis.
Unexpected frequency of the pathogenic AR CAG repeat expansion in the general population

This supports the idea that your maternal grandfather could have carried the gene for KD that your brother had. Also this gene is a little unstable in transmission from parent to child and the CAG repeat can either expand or contract and it appears expansion is more common. So I could imagine your grandfather may have had a repeat count just below the threshhold of KD or perhaps in the range of limited penetrance and did not experience significant symptoms of KD while your brother with the same or perhaps longer repeat count happened to be more significantly affected. There is also a case study of identical twins with KD who differed strongly in the severity of KD progression so it seems there are additional factors impacting disease which remain to be identified.