Reference for Bekhorot 94:27

וכי תימא מתני' שלא בהרשאה הא מוקמינן לה קמן דבהרשאה

For R'Jannai says: If the children [belonging to two women and two husbands] were identified in the beginning but in the end became mixed, they can write out a power of attorney to each other,<span class="x" onmousemove="('comment',' When they all come to inherit, each can reject the claim of the other, maintaining that he is not his brother. They therefore write out a power of attorney to each other, and approach the inheritors of the two fathers and say to each of them: 'If I am your brother, give me my share, and if this one is your brother, give me his share'.');">¹⁸</span>

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