Regarding not having the option for the pin to login anymore, that is explained here:
Specifically this section from the Roku Certification documents under section 2. Purchases (unless I am mistaken and it is a different section but this is the only thing I found relevant to it).
If you are having your sister and friend sign in to your account, even if you were doing that when the pin was available, you should just have them create Plex accounts and invite them to your server with their own account.
There are multiple benefits from this such as having their own watch history and not needing to have you log them back in if their device get signed out for whatever reason (or if they get a new device and need to sign in). There is really no reason for them not to have their own Plex accounts.
I suggest you read this support article which explains about customizing the apps:
https://support.plex.tv/articles/customizing-the-apps/
For this, read this support article (specifically under the Experience section):
https://support.plex.tv/articles/204275243-settings-plex-for-roku/
That should answer your questions.
-Shark2k
