Hello Kanaga,
As every one feels, only Dog-lovers will be able to understand the pain, its nice that you care about ur sis. I feel u should give her somtime before bringing in a new pup. When one is too much attached to the dog, its really difficult to accept another with the same extent as the previous one. But TIME IS THE BEST HEALER and definetely a new pup will brighten up her life, but after sometime.
I think ur sis must have been a good caretaker of her dog, as he lived for 13years.So I really wish another pup is blessed with having ur sis as his mother.Good Luck