Why can't I marry my Grandpa?

Amara loves to talk about marriage.  It is mostly to whoever is the sweet boy in her class at the time but occasionally it is to her Dad.   About six months ago she got past the "I am going to marry Dad when I grow up but you can still live here Mom" phase.   However yesterday she asked me, "You know, I really love my Paji, why can't I marry my Grandpa? He is not my Dad."   She figured she had found a loop hole that would allow her to get married to someone she loves and trusts and that is not her Dad.   I explained to her that you cannot marry your family at all.  Not grandpas, cousins, dads, anyone like that.  "But why?" she wanted to know.  I said the magic of marriages is when you meld two different families together - emotionally, spiritually, and physically with creating kids.  I explained that if you have babies with someone in your family, it can cause many genetic challenges.

Annika asked, "Like Down Syndrome?"  

I answered that Down Syndrome is a genetic challenge that can occur anytime with any child.   They could not understand what I was saying so I after rounds of trying to be scientific and politically correct I resorted to a very weak moment in parenting, "If you marry your own family, you can have a baby with two heads!".    Both girls got quiet.  Amara said, "That is terrible! I won't marry my family for sure!" 

Hopefully my mom and I now can be at peace knowing that Amara will leave our husbands alone.  However, for my poor sister, her boyfriend is still fair game  : )