Hello a_good_user,
Thanks for reaching out to Apple Support Communities. Upon review of your post, it seems that you have questions about the Hide My Email feature. We can see why you’d want this clarified, so let us help from here! Let’s start by reviewing the following article which may help: What is Hide My Email?
It explains: “There are two key ways to use Hide My Email: With Sign in with Apple, which lets you create an account using a randomly-generated email address directly within a supported third-party app or website. Or with iCloud+, which lets you generate as many random email addresses as you need on your device, in Safari, or on iCloud.com, which you can use for whatever site or purpose you choose.” It sounds like iCloud+ might be the way to go so that you and your sister can both have an account as long as that site allows multiple accounts from the same device.
Please let us know if you have follow up questions, so we can continue assisting. Take care!