1) As the name of this particular Salah suggests, it is meant for your parents. You
cannot pray for someone other than them. However, you can certainly make dua for
whoeever you wish in the Qunoot of the second Rak'at.
2) it is wajib on the first son of the deceased to offer his father's Qadhaa
prayers, fasts, etc.
Otherwise, it isn't Wajib (obligatory) on the rest of the family members to do it.
But if you wish to, you're free to do it for as many years as you would like to.
May Allah forgive all our shortcomings and have special mercy on all of us,
especially your Brother.