The Law Code of Yājñavalkya 3.1.37

3.1.37

yathā sagotrasapiṇḍānāṃ pretānām udakaṃ dīyate tathā mātāmahānām ācāryāṇāṃ ca pretānāṃ nityam udakakriyā kāryā | sakhā mitraṃ, prattaḥ pariṇītā duhitṛbhaginyādayaḥ, svastrīyo bhāgineyaḥ, śvaśuraḥ prasiddhaḥ, ṛtvijo yājakāḥ eṣāṃ sakhyādināṃ pretānāṃ kāmodakaṃ kāryaṃ | kāma icchā | kāmenodakadānaṃ kāmodakaṃ pretābhyudayakāmanāyāṃ satyām udakaṃ deyam asatyāṃ na deyam ity akaraṇe pratyavāyo nāstīty arthaḥ || 3.4 ||

Just as water is offered to the deceased who are sapiṇḍas of the same gotra, so the water rite must obligatorily be performed for deceased maternal grandfathers and teachers. 'Friend' means a companion; 'married female relative' means married daughters, sisters, and the like; 'sister's son' means the son of a sister; 'father-in-law' is well known; 'officiating priests' means those who sacrifice on one's behalf. For these deceased persons—friend and others—optional water is to be made. 'Kāma' means desire. 'Kāmodaka' means an offering of water through desire: water is to be offered when there is a desire for the deceased's spiritual progress; when there is not, it is not to be offered; thus there is no transgression in not performing it—this is the meaning. || 3.4