Following the request of the Bnos Tzlafchad to inherit their father, the Torah records the order to be followed in distributing a person's estate (27:8-11): sons come first, followed by daughters if there are no sons, followed by brothers, followed by a father. The Mishna in Yeish Nochlin (Baba Basra 118) elaborates on this order and includes other relatives not mentioned explicitly by the pasuk, e.g. a nephew who is the son of a sister can inherit in a case where a person passes away and has no sons, no brothers, but had a sister who passed away and left a son -- the dead sister has primacy over other remaining relatives and through the halachic process of mishmush her descendants inherit through her.
Tosfos asks why the Mishna and Torah need to explicitly tell us that brothers inherit. Once we know that a father inherits his son and sons inherit their father and we understand the process of mishmush, we can deduce that brothers inherit as well -- a brother inheriting amounts to a son collecting through mishmush the inheritance of his father received from his other brother.
R' Akiva Eiger (Shu"T Mh"K 138) reads Tosfos' answer as concluding that since the Torah and Mishna do spell out that a brother inherits a brother it implies there is a direct brother-to-brother relationship between them, not just mishmush of son to father and then father to his other son. Other Rishonim disagree and do see brother-to-brother and nothing more than mishmush in action.
A practical difference between these views may be a case where a person stipulates that if his children die before him some other party should inherit (see Kesubos 69b). No stipulation can override the right of children to inherit, and in turn their relatives and descendents can claim that right in their place. However, the Beis Yosef C.M. 243"33 writes that the children's uncle would not have a claim if the children are dead. An uncle does not directly inherit his nephew directly, but only has a claim through mishmush by virtue of being the brother of their father. Since the father stipulated that he wishes the money to be transferred to another party, mishmush would not work and the money would go to that party. The Rama in Darkei Moshe disagrees, implying that there is a direct relationship between a nephew and uncle and we do not invoke mishmush.