I don't believe an accident of birth makes people sisters or brothers. It makes them siblings, gives them mutuality of parentage. Sisterhood and brotherhood is a condition people have to work at.
But when a young lady is to be a heroine, the perverseness of forty surrounding families cannot prevent her. Something must and will happen to throw a hero in her way.
Children of the same family, the same blood, with the same first associations and habits, have some means of enjoyment in their power, which no subsequent connections can supply.