I'm so sorry about your loss.
This part really worries me:
I'd rather slit my wrists than carry this guilt.
You don't have to do either, dear heart. Guilt is, in my opinion, only useful if it stops you from doing something that you shouldn't in the moment. Or, if it helps to make amends with someone you have mistreated in some way. You tried to do that and your dad was not receptive, for whatever reason which I would bet had more to do with him than you...you should not carry guilt over what can not be altered.
One of my all time favorite sayings is something that Maya Angelou said. It is this: "You did what you knew how to do. When you knew better, you did better."
You did better, and you did the best that you could.
I like to think that whatever happens to us when we die, if we're concious of anything or become part of anything larger than our mere corporeal existence, that we know, and understand, all the things we could not during our lifetime in this confined space. I hope that my grandmother understands now things she could not while she was alive (funny, I just typed "when I was alive," and had to go back and correct... wonder what that means?) and perhaps your father does too.
As for interfering, manipulative sisters...you have also my complete understanding on that one. I have a set myself, and they make everything in life harder to deal with.
hugs...
essie