Numbers 36

"This is what the Lord commands concerning the daughters of Zelophehad, saying, ‘Let them marry whom they think best, but they may marry only within the family of their father’s tribe.’  So the inheritance of the children of Israel shall not change hands from tribe to tribe, for every one of the children of Israel shall keep the inheritance of the tribe of his fathers." vv.6-7

Read chapter 36
So we see the daughters of Zeophehad again if you remember them from chapter 27 concerning not letting their father's name be blotted out because he had no sons and asking for the inheritance to be passed on to the daughters instead.  Now the petition is by the chief fathers of their tribe, the tribe of Joseph, for  marriage inheritance of land following these daughters that it stay within the tribe.  This would cause for their land to decrease while other tribe's land increase if they married men from other tribes of Israel.  God wouldn't allow riches to one tribe at the expense of another.  So Moses commands that they marry within their father's tribe so that they don't lose land inheritance within what each tribe was allotted.  We have to note too that Moses said, "Let them marry whom they think best."  There was still free will in whom to marry just with the restriction of their father's family.  We have to remember how big families were then and if they were to marry one of their cousin they probably had 100 options!  Plus it seems a marriage ordained by God then just hope-you-choose-right kinda guy. "How could they fail to marry well when God himself directed them?" Matthew Henry)  Now this command for women to marry within their family tribe wasn't for all daughters but only heiresses.  This isn't a law for us today, but how important is it to you for believers to marry other believers (not because of earthly inheritance, but eternal inheritance)?