Your tears are about many things, I think.

Death scares all of us and when it comes close... it's scarey and sad.
It sounds like you're very compassionate and thinking of all who are being affected by this, feeling sad for them as well.
You're right, it is very sad what happened and that now there will be no time for him to make amends; he may be feeling those same regrets also.

I think that the way you can feel compassion for him in spite of his faults, says so much about you as a person. You are a generous, compassionate, and caring person and I think that's very admirable and touching.

You don't have to feel guilty: it's okay to not like how he treated your sister and the kids at the same time as feeling sorry for his being deathly ill.