First, I am so sorry for this news for you and your family. I am not sure on slowing the process of the visa however I have experience with pancreatic cancer. I do not want to be a downer or the bearer of bad news but I went through this less than two years ago with my Dad. He was diagnosed in December and passed by July and that was with aggressive chemo. Of course everyone is different. My suggestion is to have your fiance attend the medical appoints with her Mom and discuss options and what they feel is the timeframe. Explain the situation you are in with the visa to the doctor so they can explain what the different stages of this cancer can look like. Again, I'm so sorry for this terrible news.