[Update] My [34/M] sister [39/F] is terminally ill. I wasn't going to see her before the end, but now I'm having second thoughts.

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Welp, I did it. Got back like an hour ago. Had to update all of you. First.... thank you for all of your advice.
Almost immediately after consulting ya'll, I called my sister's husband to do what some of you had recommended and to ask what it is she wanted to talk about. I called and my sister answered (It was a land-line. Who has a landline anymore??). I didn't realize I would be thrust into talking to her straight away. She was very quiet, really weak and asked that we meet. I agreed and went this morning (I took the day off and brought breakfast foods).
I hadn't seen her, in person, in years, so in my mind she was about 8 feet tall and mean. She's about 4'11 now and a tiny wisp. She was mostly bald, very, very pale and had been placed in a chair in the living room. She doesn't look anything like I remember, much older but also very different. She looks a lot like our grandmother, which was interesting to see.
We had a really, really awkward conversation for the first 10 minutes, that sort of conversation where you know you need to discuss something, but one, or both, people are trying to avoid the subject. She sipped some juice and we ate pastries I had brought. Her daughter came out and introduced herself, she knew who I was almost instantly and was very polite. She helped me eat the pastries and enjoyed the juice.
My sister made a full, unreserved, apology. She outlined that she had been abused by a distant relative (something I had pieced together/my mother let me know) and had a lot of problems. She apologized for what she did and how it impacted the rest of my life. She blames herself for things that I did in the wake of the attack - I gave up a lot of interests I deemed "nerdy" and wound-up in the military for a spell, which I probably wouldn't have done - and wanted me to know she was sorry. I accepted her apology in a sort of "no need to apologize" kind of way. I'm not sure why I said that, but it seemed only right at the time.
I met her husband who gave me a picture frame he made from wood, it's very beautiful and I'll use it to frame a family picture. Her husband isn't what I expected at all - very meek, quiet, and seems to be a genuinely nice guy going through what I imagine is a horrific time.
My mother called during our meeting and had no idea I was going to see her, so she was a bit overcome emotionally. That I guess is what really kind of bugged me. I was really caught-up in how mad I was at my sister that I forgot that my parents have to bury a child and that a child is going to lose her mother - no matter what I thought of her mom. I felt a bit guilty about the whole thing after the fact.
My wife was really concerned for our niece and has suggested that we arrange a dinner on Saturday so that our kids can meet their cousin and hopefully forge something so that she has a bigger network of people for when the inevitable happens.
All-in-all, it was positive. I can know she was apologetic and that I won't have the nagging question when she passes. My wife wants to do right by our niece and my mother seems to be incredibly happy that we hashed things out. She called my wife and told her she had been hoping and praying there would be a happy resolution. So that's a positive turn.

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tl;dr: Wound up seeing my sister. She's not doing well. We had an awkward conversation and she apologized where I said she didn't need to. I met my niece and her husband. Our meeting has made her, my parents and sister really happy. It's made my wife want to get to know her niece. I got an unreserved apology and feel good knowing I got closure and didn't let this linger until it was too late. Feel bad for those around me, especially my niece.


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