The Law Code of Yājñavalkya 3.1.36

3.1.36

evaṃ mātāmahācāryapretānām udakakriyā |
kāmodakaṃ sakhiprattāsvasrīyaśvaśurartvijām ||

So is the water rite for deceased maternal grandfathers and teachers; / optional water is for a friend, a married female relative, a sister's son, a father-in-law, and officiating priests.