Kautilya's Arthashastra 3.6.13

3.6.13

nānāstrīputrāṇāṃ tu saṃskṛtāsaṃskṛtayoḥ kanyākṛtakṣatayor abhāve ca ekasyāḥ putrayor yamayor vā pūrvajanmanā jyeṣṭhabhāvaḥ //

With regard to sons of many wives:--

Of sons of two wives of whom only one woman has gone through all the necessary religious ceremonials, or both of whom have not, as maidens, observed necessary religious rites, or one of whom has brought forth twins, it is by birth that primogenitureship is decided.