Why would I want to use something that makes someone's bitcoin disappear??
Because the issue here is likely user error.
There are millions of users of Trezor devices, and issues such as this one are very rare. This is not the same as blockchain.com wallets, where issues are incredibly frequent and are far more likely due to bug or flaws in the code.
If the money showed up in their trezor but then it disappeared how do you explain that?
Using the wrong Trezor device. Using a different seed phrase. Using a passphrase. Using a different derivation path. There are multiple possibilities, and it sounds like OP's sister doesn't know what she did or how she did it, so user error is the most likely issue here.