Amy, if you look back in threads you'll see as a mom I faced this same question when my then 5 year old's snake got loose and died. It's the first reaction to protect young kids from distress but in the end, it's not the right reaction. Instead of switching snakes, help your sister take this snake to a vet, figure out what is causing the regurges (health or husbandry) and either help her bring the snake back to health or help her deal with it's passing. In the end, you'll be helping her face life's realities better and be setting a fine example as the older sister.