Reference for Sanhedrin 56:4

ובעל אמו הוא ובנו וחתנו: בנו היינו אחיו

If so, [the difficulty remains,] he [i.e., his step-father's son]is HIS BROTHER! — He [the Tanna] enumerates both a paternal and a maternalbrother.

Jastrow

Jastrow