I lost my grandmother and dad to Alzheimer's. I feel like it's probably in my future, as well. It's a miserable disease that makes me question a lot of end-care practices and my wife & I are absolutely going to have some strategies in place, but to answer your question we already have 3 beautiful children that I can't imagine life without (pretty sure there's an Alzheimer's joke in there, somewhere). I certainly don't want to put them through what I saw with my Dad, but I also wouldn't want to have missed out on all the wonderful experiences I've had/will have with them on account of a "what if".
Ultimately, the choice is yours to make but you're clearly putting a lot of worthwhile thought into it and I'm sure whatever you decide will be the best choice for you. I'm sorry about your dad & sister, and if you ever need someone to talk to I'm here.