Two-Family House by Lynda Cohen Loigman
Two Jewish brothers and their families live in a brownstone. Abe lives upstairs and has sons, Mort lives downstairs has daughters. They think they have had as many children as they are going to have until both their wives, best friends, Helen and Rose become pregnant. So it happens that Helen and Rose deliver at the same time during a blizzard when the men are gone. It is the start of a secret they keep for thirteen years that changes one of the women so terribly she can no longer stand the other. What pact did they make that night that would forever change their lives?