If i'm good for the Animus, wouldn't my grand/dad/brother be as well?
After all, it's the same bloodline, isn't it?
Or is the genetic memory stronger in certain offspring/generation than others?
As long as you, your dad, and your grandfather share the same ancestor (i.e. it's not an ancestor from your mom's or grandmother's line) then yes all 3 of y'all would be as good in the Animus.
Genetic memory is the same vertically on a family tree (you have the same genetic memories of your great-grandfather, as your dad, and your grandfather) the differences come from the people horizontal to you on your family tree. While you and your brother could view your dad's genetic memories since genetic memories are passed on at conception whichever brother is younger has more memories since he was conceived later.