It's ages since I read "War and Peace" (life is too short to read it twice), but I recall the odd situation where a young woman is in love with her cousin and that's OK, but there are also a pair of siblings, each in love with one of another pair of siblings, and when teh first two get married, that stops the other two from doing so. Or, to put it in JA's terms, the marriage of John and Isabella would have stopped the marriage of George and Emma Knightley.