It is narrated from Amr bin al-Sharid that Abdullah bin Abbas (may Allah be pleased with them both) was asked about a man who had two wives; one of them suckled a boy and the other suckled a girl. May the boy marry the girl? He said: No, because their insemination, or the one who made them pregnant, is one and the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15617