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In the MilindapaΓ±ha the benefit of dedicating the merit of a gift to one's deceased relatives is limited to those who have been reborn as just one of the four sub-species of peta. It's explicitly denied that there can be any benefit to those reborn in hell, heaven, as animals or in the other three peta sub-species. Curiously nothing is stated one way or the other about those reborn as humans.
Rhys Davids' translation:
https://legacy.suttacentral.net/en/mil6.3.4
I.B. Horner's: Pubba-peta-panha.pdf
Above post appeared in Dhamma Wheel.
https://dhammawheel.com/viewtopic.php?f=19&t=32595&hilit=Merit+transfer