My mother is one of six siblings and four of them have dementia. My mother has Dementia with Lewy Bodies, her sister has Alzheimers, a brother has Vascular Dementia, and another brother died with dementia (not sure what type, he developed it after a severe blow to the head).
The other two siblings - one currently has a brain tumour and the other passed aged 62 from alcoholism - he could have developed dementia too if he had lived longer, who knows.
Her parents never had anything like that, although her mother passed aged 65 from heart problems but her dad lived to be in his 80s and he was fine.
No-one on my dad's side has had dementia, so I hope I take after that side rather than my mother's!