<human>: Can you give me a shortened version of the text?

My father's father left his wife and only son (my dad) back in 1953 for another woman, and they moved across the country. He came back to see my father once (he left when my dad was 3, he was 6 when he returned), and has had absolutely no contact with us since. 

So my grandfather's sister ended up keeping in touch with my great grandmother (long story, but they were neighbors) up until right before I was born. We found out that my grandfather had three children (2 boys and a girl), and that he passed away in 1998. 

I am contemplating reaching out to my long lost family for a few reasons. First, I would like to know what he was like, and I'd like to hear another side of the story. Second, I want to know why he died at a relatively young age, in case of any genetic diseases I should be aware of. 

Has anyone else done this? How did it work out for you? How did you go about finding them? 

What if they have no idea that my father exists, and I disrupt their lives, telling them that their father/grandfather was a neglectful person who abandoned his first family. 

Here's the real kicker though--I really want to find my grandfather's sister. She was adopted when my grandfather was 18. He was an only child up until that point--why adopt? The crazy part is, she looks EXACTLY like my father, so we have theories that she was the result of a baby born out of wedlock between my grandparents.
<bot>: I want to find my grandfather's "other" family to learn about him, not sure if it's worth it.