Sorry to hear about your loss. That is quite unbelievable so many in the family gone in such a short time.
There's maybe not a specific reason for this. It could be genes I guess?

Brain conditions are extremely rare at such a young age.
Things like these certainly make me believe things do NOT happen for a reason whatsoever, I can't really understand why so many people live by that rule. Most of us on the board here will be in the top 1-3% of most affluent people in the world. There's no reason for that either in my opinion.