I see Only 2 option now:
1. 24 seed generated completely Randomly and Both device got the same seed. But its very unlikely
2. Ledger use pre generated 24 seed and probably due to an error both device got the same 24 seed. This senraio is more likly to me.
But again I have no any evidence.
Both sounds unlikely, where 1st option is more than very unlikely. Anyway, since Ledger these days have poor reputation, you might want to do some research and then suggest your sister to buy better and more reputable hardware wallet.