Is it reasonable to expect my sister to reimburse us for this sweetheart deal that we were not a part of?

LW3… No??? WTF is wrong with you. Your grandmother died (recently?) and you're mad that she left some extra money to your younger sister who is going through a painful, expensive (not to mention, emotionally difficult) medical procedure to offset the costs.

It sounds like you got a pretty nice chunk of change anyway even if you were expecting $3,000 more. You should be grateful you got anything.