"When brothers dwell together": to exclude a brother who was not in his world (i.e., who was not concurrently alive). From here they ruled: If there were two brothers and one of them died and another one was born, after which the second brother performed yibum with the wife of the first, after which he died, then the first wife (i.e., the wife of the brother who died first) is exempt (from yibum) by reason of her being "the wife of his brother who was not in his world," and the second wife (i.e., the wife of the second brother) by reason of her being her (the first wife's) tzarah (co-wife). "together": in inheritance — to exclude maternal brothers.
Because we find in arayoth (illicit relations) that a maternal brother is equated with a paternal brother, I might think that here, too, this is so. It is, therefore, written "together" (in inheritance), to exclude his maternal brother.