הלכה על סנהדרין 3:4

And from the laws of this commandment is that which they, may their memory be blessed, said (see Mishneh Torah, Testimony 13:1) that the disqualification of being related from Torah writ is only with the family of the father - and they are the father with the son and with the son of the son; and the brothers from the father with each other, and their sons with one another; and there is no need to say, the uncle with the son of his brother. But with the other relatives from the mother, and so [too] the relatives from the side of marriage, their disqualification is [only] rabbinic. And this reasoning is taught in the last mishnah in the chapter [entitled] Zeh Borer (Mishnah Sanhedrin 3:4), which is all the extant law, according to the opinion of a few commentators. But from [the commentators], there are [other] great ones - sages and wise ones - that decided [that] even all of the sibling relatives from the mother, and so [too] his brother-in-law and step-son and all that are not fit to inherit, are [all] disqualified from Torah writ, [just] like the [corresponding] relative on the father's side - as they learned [to include] them in the Gemara, from the inclusion of the verse, [based on its mentioning the word,] fathers, fathers twice. And their proofs are in their books - merit, my son and you will distinguish the truth.