Oh, Sonjah, so sorry for your loss.There are so, so many reasons and ways buns can die suddenly that it ishard to make the diagnosis that you are asking for. You don'tsay how long your sister had the buns. If you had them threeyears, and she another two or three, that would even make natural deathhighly likely, the "average" life span of our domestic rabbits is fiveto ten years, with odds more toward the short end of that statistic.
Some or our buns free range pretty regularly, so when I say that yourbuns enjoy and enjoyed their "free" life, I know I am correct basedupon the reaction of our bunnies to free ranging opportunities.
We also have house bunnies, but that is more for our ownenjoyment. I would never equate a house-bound life with thejoys and contentment of a "protective" free rangingenvironment. Our back yard becomes "Binkyville" when the bunsare left to their own free ranging resources, which I'm sure you willunderstand.
Rest assured that you have given your buns a very fruitful, if not long life. I am sure they will love you forever.
Buck