I have to occasionally use Windows 10, Windows 11 and macOS.
Like you, I never store anything directly inside my system. Never have, even before I switched to Linux.
As a general rule, anything I will want to access on all 3 systems, I store on a USB or external drive in exFAT. This is so I can easily plug it in to whatever computer I'm working on, but I could just as well have an internal drive with a data partition formatted in exFAT if I wanted - say if all 3 systems were contained within one computer.
On this computer, my current favorite, one of my drives is split between an NTFS partition and an EXT4 partition. The NTFS partition is where I bare metal boot my Win 10 and Win 11 virtual hard disks from. Since these are virtual disks and not installed on the computer, the NTFS partition can be considered a data partition. Whatever I am doing inside either of Win systems, I save outside the running system in the same NTFS partition it's running from. This means I could access it from my Linux instance later, but I won't. If it's something I will need to access between Linux and Mac too, I save it to my exFAT filesystem.
This way later, when I am on the same computer but I am in my Zorin virtual disk, I can access what I had been working on in Windows from the exFAT filesystem, and I never have to mount the NTFS partition with Linux. Just because so many people say it's a bad idea, so I avoid it.
Working between different filesystems you also have to be aware of some of their differences, like case insensitivity in names, and how their permissions work or are ignored between different operating systems.
So if you had an exFAT partition, surely you could set up both your Windows and your Linux to ignore that partition and restrict access to some users. You would just have to do it from within each system as the admin, and not rely on the exFAT partition take care of these administrative tasks itself. So for example I assume your mother can boot from a Live ISO of some sort where she would then be able to see the data partition and mount it and peruse its contents. But if she is using any sort of operating system where you are the administrator, you should be able to hide the partition and also prevent access (2 different things).
EDIT: Just to avoid confusion, exFAT does not support permissions. Which is quite a blessing when you are sharing things between Linux, Mac, Windows and sometimes FreeBSD.