Responsa for Sanhedrin 9:13
ואי בעית אימא
to Babylon. May he, decide in matters of ritual law?' Rabbi answered: 'He may. May he decide monetary cases?' — He may.' 'May he declare firstborn animals permissible [for slaughter]?'<a rel="footnote" href="#5b_14"><sup>9</sup></a> — 'He may.' When Rab went there, R. Hiyya said to Rabbi: 'My sister's son is going to Babylon. May he decide on matters of ritual law?' — He may. 'May he decide [monetary] cases?' — 'He may.' 'May' he declare firstborn animals permissible for slaughter?' — 'He may not.' Why did R. Hiyya call the former 'brother's son' and the latter 'sister's son'? You cannot say that it was actually so, since a Master said that Aibu [Rab's father] and Hana [Rabbah's father], Shila and Martha and R. Hiyya were the sons of Abba b. Aha Karsela of Kafri?<span class="x" onmousemove="('comment',' In Babylonia. Hence Rab was also the son of R. Hiyya's brother's. ');"><sup>10</sup></span> — Rab was also R. Hiyya's sister's son [on his mother's side], while Rabbah was only his brother's son. Or, if you prefer, I might say he chose to call him sister's son'