Reference for Sanhedrin 55:25
ואכתי הוה ליה שלישי בשני ורב אכשר שלישי בשני
Rab said:<span class="x" onmousemove="('comment',' To understand Rab's statement and the others that follow it is necessary to give some explanation of affinity and consanguinity in Talmudic law. Relationships between persons are divided into two categories: (a) relationships between persons governed by the ties of consanguinity, i.e., persons of the same blood either lineally or collaterally; (b) relationships through marriage, i.e., affinity. And on the principle that man and wife are considered as one, the relatives of the one are related to those of the other by affinity. Again, the rules by which kinsfolk are excluded from bearing testimony for or against each other affect only certain degrees of relationship, e.g., relatives in the first degree, such as father and son, or brothers may not testify for or against each other; relatives in the second degree may not testify for or against those of the first degree. e.g., a nephew for his uncle; relatives in the second degree may not testify for or against each other, e.g., first cousins. On the other hand, relatives in the third degree may testify for or against relatives in the first, e.g., a grand-nephew in respect of an uncle (according to Raba in B.B. 128a, in opposition to Rab's opinion here); and relatives in the third degree may testify for or against relatives in the second degree, e.g., first cousins for second cousins (Rab agrees with this opinion, but not R. Eleazar.) It should be noted that the ineligibility is mutual. ');"><sup>23</sup></span>