I can reproduce this behavior also on Monterey. Reply To All behaved as you described.
Based on Luis' reply, it seems consistent between Monterey and Ventura.
I don't know if this is a bug or a "feature" that is working as intended by Apple. I can see the logic either way. Apple might have done this to fit the flow of how most people handle emails, which is have separate email accounts under separate user logins. I can verify that then all recipients show up in a Reply To All "To" list.
I think your setup with one user login but separate user email accounts (under that same user) going to separate people (users) is not common. I think Apple is assuming that different people use separate user logins on the Mac, and one login with multiple email accounts means one person, so Apple might have designed this not to send duplicate emails to the same person. Although in your case it is two separate people.
It could also be a bug. I'm not sure there is an easy way to find out if this behavior is what Apple intended, but I am guessing that it is what Apple intended.
The only solutions I see for you are: (1) monitor your sister's email under a different email app, or via a web based interface; (2) create a separate login for your sister on your Mac, you can use rapid account switching to quickly switch to that account to check that email, then rapidly switch back to your normal account; (3) when using Reply To All, add manually the one email address that is missing.
