PAW, except for minor details your story is so similar to mine, it's freaky. There's great advice here already. I can only tell you what I did. I dropped everything, settled the kids and flew out there. I sat at his bedside and didn't say much, and tried to be as helpful as I could to the rest of the family...stepmom, two half-siblings, his in-laws, coworkers, friends. I was with him as he passed away. Then there was a flurry of activity...funeral arrangements and stuff. I went home that Saturday, missing the funeral and everything, but wild horses couldn't have kept me from going home that Saturday. We had a memorial service out here for him two weeks later, which was well-attended even though he hadn't lived in the area for over 25 years. My Mom and I went to it together. That was kind of weird in itself.
There was a time when my father and I were alone in the room and I was just sitting there, patting his arm, with the TV on...it was peaceful and he did seem to be....I don't know, more settled and content for the moment, so that I felt his presence and that it was the right place to be. And it was comforting to me. That's about the last memory I have - he died later that day.
This was in July, right around my birthday in fact, so I'm still shaky about it in some ways and I wouldn't say to you, "you have to go to him." You have your stepmother's emotional issues to contend with too, and I really didn't, and I guarantee you'll be seeing a lot of each other. Just do the best you can.