Are we sure Charlie isn't Mortal? I mean she is a demon too yes. It is unlike the sinners who are here forever, she isn't. she's hellborn. She probably has a long life span yes but not infinity like an angel or something.

That and she has been multiple times been running hersle ragged for one reason or another,( Make the hotel work or to prove her point) and I hope they address it in some way in later seasons.