'have' means "up to now", while 'had' means "up to then", 'then' being a, usually specific point in time in the past.
Both would work because the family members praying for the deceased faimliy members would obviously include all of them even if a 'had' was used.
But usually in English when we collectivize something/a group of people in this fashion, it often has a current relevance, so we normally choose the "up to now" collocation using 'have + PP'.
It means we do this everytime we say the 5th silent prayer.
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