Ok - aside from the fact that sharing accounts likely breaches the terms of service … there are issues with trying to have two linked accounts on the one device, as in the process of trying to log in and out and switch between them, you can easily overwrite one with the other, or accidentally change how they are linked.
if you and your brother can both still access your accounts from your own devices and you haven’t overwritten either of them, then I would call that a win and not push your luck by trying to have multiple accounts signed in on the same device.
if you do need to help him out by maintaining his account when he can’t log in, then I would suggest accessing it on a seperate device to the one you have your main account on … for example, I have my account on my phone and iPad, and my second account I only ever access from my partners old iPad that she hardly ever uses.
i used to try and access it from my iPad by switching accounts etc, but ran into similar issues with it not always linking or logging in.
So yeah - in theory it should work, but in practice it is way to easy to accidentally break everything 😬