Commentary for Numbers 36:12

ממשפחות בני מנשה, seeing that they had realised that it was G’d’s will that the land should not be transferred from one tribe to another, even though He had given permission for them to choose a husband from any family of their tribe in accordance with their wishes, they chose the sons of their uncles over other families of their tribe as these were more closely related to them ותהי נחלתן על מטה משפחת אביהן, so that there was no transfer of any land to another tribe by reason of their marriages. In fact, no land was reassigned to another family within the same tribe.