If you are both sure this is the love of your life you should go for it.
Your family may or may not come to terms with it but from what you say you have no alternative.
And also,around the same time, my grandma passed away and I pushed everyone away.
We talked sometimes, just to catch up and see how the other was doing. They're not acting like family, they're acting like a couple. Not because I still had feelings for him or because I thought he was "mine" or anything, but because my cousin and I had previous issues. Now thanks to the both of you, I won't feel comfortable bringing any future boyfriends around my cousins or friends. Thank you both for proving to me that no matter how big my heart is, there are still people out there who will do whatever they can to tear it to pieces.
My best friend took a long time to accept our relationship.
What's the best way of breaking this news and how do Alison and I deal with the fallout?
The only thing that really need concern you is whether there is any genetic risk to the children you may have. Consult a properly qualified expert then act accordingly. Otherwise you are adults and as you are doing nothing wrong your prime responsibility is to your own happiness.