In my opinion that is a perfectly sound solution if it works for you and your sister. I think the difference is that when you give your login details to someone else while you are still alive, in theory they could get into your device any time from then on and so you must have complete trust in them NOT to abuse that privilege. With Apple's Legacy provision that would not be possible until after your death, and then access to your device is (probably) less of a concern to you.
In my case I have set up my phone with the Legacy settings so my wife can get into my iPhone when I shuffle off, but in the meantime I have my login info written down in a place only she and my daughter are aware of, so I am doubly covered.