The Talmudic passage in Sanhedrin 5a discusses the familial terminology used by R’ Ḥiyya to describe his relationship with Rabba bar Ḥana and Rav. The question discussed is why R' Ḥiyya refers to Rabba bar Ḥana as his “brother's son” and Rav as his “sister's son”, despite both being his brother's sons: both sages are sons of R' Ḥiyya's brothers, as they are listed among the sons of R’ Ḥiyya's father, Abba bar Aḥa Karsala from Kafrei.
The resolution provided by the anonymous Talmud is that Rav was his sister's son in addition to being his brother’s son, because of an intermarriage within the family: R' Ḥiyya’s half-brother had married R' Ḥiyya’s half-sister, thus making Rav both his brother’s and sister’s son. Conversely, Rabba bar Ḥana, being only his brother’s son and not related through a similar intermarriage, was simply referred to as such. This distinction in terminology used by R’ Ḥiyya emphasized the different familial connections between R' Ḥiyya and the two sages.
Sanhedrin.5a.12 (numbering added by me):
מ"ש למר, דקא קרי "בן אחי", ומ"ש למר, דקא קרי "בן אחותי"?
וכי תימא הכי הוה מעשה:
והאמר מר:
איבו,
וחנה,
ושילא,
ומרתא,
ורבי חייא,
כולהו בני אבא בר אחא כרסלא מכפרי הוו?!
רב - בר אחוה, דהוה בר אחתיה,
רבה בר חנה - בר אחוה, דלאו בר אחתיה.
What is different concerning this Sage, Rabba bar Ḥana, that R' Ḥiyya called him: My brother’s son, and what is different concerning that Sage, Rav, that R' Ḥiyya called him: My sister’s son?
And if you would say that this was the situation: Rabba bar Ḥana was his brother’s son and Rav was his sister’s son,
but doesn’t the Master say:
Aivu, Rav’s father,
and Ḥana, the father of Rabba bar Ḥana,
and Sheila,
and Marta,
and R' Ḥiyya,
were all sons of Abba bar Aḥa Karsala from Kafrei? Consequently, Rav would also be R' Ḥiyya’s brother’s son.
The Gemara answers:
Rav was his brother’s son who was also his sister’s son, as R' Ḥiyya’s half-brother married R' Ḥiyya’s half-sister;
while Rabba bar Ḥana was his brother’s son who was not his sister’s son. Therefore, he referred to Rav in a manner that emphasized the additional relationship.
Graphing the family tree, according the explanation of the anonymous talmud (stam)
Hebrew Wikipedia has a graph, mapping out the family tree:
תבנית:עץ משפחת רבי חייא – ויקיפדיה
This Hebrew Wikipedia template is used in a few entries, for example this one:
אבא בר אחא בר סלא מכפרי – ויקיפדיה
However, since this graph includes many additional family members, it’s a bit confusing for our purposes. To this end, I made a focused family tree using the technique of ASCII art:
Alternative Theory
The anonymous Talmud clearly understands that all the children (b’nei) of Abba listed by the anonymous “Master” (מר) are males. But it seems likely that Hana (חנה) and Martha (מרתא) are females. These are clearly female names, and are used as such everywhere else.
For more on these names, see here:
Hannah (name) - Wikipedia. The origin of this name is the biblical Hannah, mother of Samuel.
Martha (given name) - Wikipedia:
“Martha is a feminine given name (Latin from Ancient Greek Μάρθα (Mártha), from Aramaic מרתא (Mārtā) "the mistress" or "the lady", [..] feminine of מרי [or מר - mar] "master").”
The first people recorded with this name are 1st century women: the talmudic Marta bat Baitos, and a woman mentioned in the New Testament Gospels
These names are mentioned as personal names for women elsewhere in the Talmud:
The afore-mentioned Marta bat Baitos, in Gittin.56a.11:
מרתא בת בייתוס עתירתא דירושלים הויא.
Marta bat Baitos was one of the wealthy women of Jerusalem.
A woman named Hannah, married to the known talmudic sage R’ Mani/Mana, in Taanit.23b.13:
אמר: לא מיקבלי עלי אינשי ביתי.
אמר ליה: מה שמה?
חנה.
תתייפי חנה.
ונתייפת.
אמר ליה: קא מגנדרא עלי.
אמר ליה: אי הכי תחזור חנה לשחרוריתה,
וחזרה חנה לשחרוריתה.
R' Mani said to his teacher: The members of my household, i.e., my wife, are not acceptable to me, as she is not beautiful.
R' Yitzḥak [ben Elyashiv] said: What is her name?
R' Mana replied: Ḥana.
R' Yitzḥak declared: Let Ḥana grow beautiful,
and indeed she grew beautiful.
After a while, R' Mani said to R' Yitzḥak: She acts haughtily toward me, due to her great beauty.
He said to him: If so, let Ḥana return to her homely appearance,
and she returned to her homely appearance.
Interesting as always. Is the "alternative" theory yours? Also, I'm not clear on the two half-siblings. Was the marriage free of prohibition as a result? Or was it still problematic? was it the problem or the uniqueness of the situation why it merited discussion in the Talmud?