Surrogate Pregnancy Clarification

Relating to this post, in Parshas Vayeitzei.

When some rishonim mention that the fetuses of Yosef and Dina were switched, the outcome is that the surrogate mother is the mother. According to those who use this midrash to explain that Shimon married Dina, they would have to hold that on some level the surrogate mother is the mother - because Yosef is the son of Rachel and Dina is the daughter of Leah - but they'd also have to hold that for some things the genetic mother is the mother, otherwise, Shimon and Dina would be full siblings and not have been allowed to marry.