By asking the question, she opened herself up it being our business.
The 15 year old is old enough to carry her own Epi-pen. At 15 it's quite possible that they might not be together every waking moment, and if the OP has the Epi-pen in the ladies room when the OP suddenly feels her throat closing up, it's not going to do anyone any good, right?
Once a child is old enough to understand when it needs to be used and how to use it, they should be carrying it themselves. Like I said, it won't do teh sister any good if the OP has it and isn't right there when the sister needs it.
Anne