By helping to ensure the survival of a brother or sister, they increase the chances that many of their genetic characteristics get passed on through their brother or sister's offspring.
(they; increase; the chances that many of their genetic characteristics get passed on through their brother or sister's) (many of their genetic characteristics; get; passed on through their brother or sister's) (many of their genetic characteristics; get passed on; )
they many of their genetic characteristics many of their genetic characteristics
increase get get passed on
the chances that many of their genetic characteristics get passed on through their brother or sister's
passed on through their brother or sister's