Since when is a woman who inherits forbidden to marry into a different tribe than that of her father?
1.
Rashi (on Pasuk 27:7), Ramban and Targum Yonasan: The prohibition of marrying into a different tribe ("Lo Sisov Nachalah")
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was restricted to that generation exclusively.
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Ramban (citing Bava Basra, 120a): B'nos Tzelofchad were the only daughters in that generation who inherited, which explains why they were the only family to lodge a complaint.
2)
Why was the Torah not similarly concerned regarding an heiress who was already married, or a woman whose father or brothers would die later (leaving behind no sons) after she already married into a different tribe?
1.
Ramban #1: The Torah was only concerned about something that was rectifiable, which those cases were not. And it did not want to change the laws of inheritance in such a case by prohibiting her husband or her son from inheriting her.
2.
Ramban #2: Pasuk 5 and Pasuk 6 are talking about two different Mitzvos.
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Pasuk 5 is a prohibition against an heiress marrying into a different tribe, whereas Pasuk 6 teaches that if a woman married into a different tribe and then inherited property, her sons or husband do not inherit her.
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