Well, there are threads and some info online about this or similar things... maybe do some googling.. There was a show on TV in our country a few months ago too... You could briefly mention you read an article about it/saw something on TV/online, and see her reaction? (It's good to have some info to talk about then - she may ask: 'What did they say'?)
My dad teased me that my coz and I would be 'perfect for each other', as we have some similar interests, but I don't share this.. In some ways maybe, in some ways no.. So even if you get along well, she may not share this view.. Maybe try to find someone who is similar to her or to your view of her? (Maybe you don't even know the real her and just like the idea of her? Especially if you see each other rarely, just for holidays or such?) Think about what things are important to you and see where she stands on those things..
Some people do get married or have relationships, I think the genetic risk for any kids would probably just not be very good.. (Though there are assessments for that too, if I remember it right...)
You may be just 'accustomed' to her due to familiarity, so it's easier to talk to her.. If you take time to get to know other girls/women and spend time with them on a repeated basis, you may realize that you can feel 'comfortable' around them too.. Do you see other women/girls on a repeated basis, eg through volunteering or dating or going clubbing/dancing etc?
I think for Christmas, it's better to be neutral and observe.. See what she's like.. Get to know the new her, find out if she's changed from last time you saw her... what she's interested in etc. Maybe she even has a boyfriend or a crush on someone? Or maybe you may find someone inbetween too?!!