Put something in between.
Man ---- Wife is a one-to-one relationship. (Well, some of the time, anyway )
Brother ---- sister might be a many-to-many relationship. Put something in between; you can have a "sibship" with one brother and one sister and you interpose that between brother and sister
Brother ---- sibship = one-to-many: one brother many have several sibships. Sibship ---- sister = many-to-one: several sibships may lead to one sister.
Brother ---- sibship ---- sister is now no longer a many-to-many.