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user15464
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According to Gemara Sotah 10a as explained by Rashi, Tamar wasn't even related to Yehuda as a daughter in law once Er and Onan died as she was an orphan married by her mother which is not a kiddushin Mideoraita, otherwise Yehuda would have been forbidden to marry Tamar as stated in Vayikra 18,15: ערות כלתך לא תגלה. So Yehuda asked her if she was an erva (i.e daughter inlaw) to which Tamar responded no because she was never able to marry properly as her father was already dead and her marriage was a Kiddushei taus:

יתומה אני - והייתי קטנה והשיאוני אמי ואחיי ואין נישואין לער ואונן כלום לאסור עליו משום כלתו ואין זכות לאם ולאחים להשיא קטנה אלא האב זוכה בה

So since a father in law can't do Yibbum, Yehuda married her from scratch as she was never considdered married to Er or Onan, Peretz was Yehuda's son not Er's son.

user15464
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