Reference for Sanhedrin 150:15
ועוד מה היא לא חלקת בה בין אמה לאם אמה אף הוא נמי לא תחלוק בו בין אמו לאם אמו
What then is the meaning of 'lower'? Lower in [gravity of the] prohibition.<span class="x" onmousemove="('comment',' I.e., 'the upper' or higher prohibition is that of his mother-in-law, his more immediate relation, whilst the prohibition of her mother, as also of his father-in-law's mother, is regarded as 'lower', i.e., weaker, as they are a generation further removed. Hence this is its meaning: Whence do we know that his mother-in-law's mother and his father-in-law's mother, whose relationships are lower (i.e., further removed, and consequently weaker) than his mother-in-law's, are treated as his mother-in-law? — It is derived from his wife's daughter: just as in the latter case, the 'lower' relation is as the 'upper' (stronger), i.e., his wife's daughter's daughter is as his wife's daughter, though more distant; so here too, his mother-in-law's mother is as she herself. This deduction is in respect of equal punishment. The second clause is explained by R. Ashi as Abaye, as referring to the prohibition. ');"><sup>15</sup></span>