Is pretty old to be an egg donor. Usually 35 is the cut-off age, my clinic only used donors under 32. Have you considered what you will do if your sister doesn't pass the screening tests or if even if she does pass the cycle is unsuccessful?
Personally I would think long and hard about having my sister as a donor. I understand that it is nice to retain the genetic link, but I would always prefer using a stranger with whom I can have a clear-cut arrangement with no implied obligations over a family member for something like this.
How did I feel when I was told I needed DE? Devastated. I have not told my family and have no intention of doing so, it's none of their business. I will tell my genetic and DE children when they are old enough to understand.
Wising you the best