M.O.
I'm so sorry for your loss. Guilt is a natural, inevitable, human reaction, but it doesn't sound like you did anything wrong at all. It sounds like her condition took an unexpected turn for the worse, and how could you foresee that? Even her own daughters didn't see it coming.
Even more importantly, toward the end of life, people look at their lives retrospectively. Your dad's girlfriend was probably just very glad that your dad had wonderful children and that they loved and appreciated her. In-person visits can often be exhausting and overwhelming -- that's why hospice nurses and other professionals usually recommend "Come visit, but keep it simple and don't stay long."
Rather than focusing on your presence or absence then, I really, really recommend focusing on what your dad needs now. Seriously ill people don't always need visits, but grieving widowers DO. Your important role in all this is beginning right now. Spend as much time with your dad as you can. I'm sure that's exactly what his girlfriend would have wanted, and it's probably the most important way of all to honor her memory.
In sympathy,
Mira