conversation with a four year old

Conversation with a four year old:
Z: "If you die before me, who will look after me?"
Me (trying to be as comforting as possible): "You will be a grown-up and be able to look after yourself.. you will be like Daddy and you might have your own children to look after"
Z: "will i be a Daddy? and who will be Mummy? Do boys marry girls?"
Me: "Yes that's right.. so you could marry Suzy or Milly or Jane"
Z (fascinated by this idea): "Who else?" "Who else?" .. until we had been through most of his friends.. "or M!"
Me: "Well no, you're not supposed to marry your sister. You marry someone from a different family with a different mummy and daddy"
Z: "But Aunty Tri married Uncle Nick and he's from the same family"
Me: " No he is in the same family now because they are married.. but he has a different family and a different Mummy and Daddy to Aunty Tri."
Z: " where does he live?"
Me: " He lives with Aunty Tri but when he was a boy he lived with his Mummy and Daddy in a different house"
Z: "I think I WILL marry M"
Me: "well you're not really supposed to marry your sister."
Z: "well, I will tell the police."