By default their app cannot though because Android uses proper sandboxing and gated API access. So you actually have to give the app location access, access to your messages and access to your photos.
Well, unless you use one of the many crappy Android devices that never get security updates, are running old kernels, old vendor security patch levels, miss all Android security patches, except applying the backported security bulletins every three months (1-2 months late). Yet, Google is happy to certify them as Android devices.
It was never about security, it is about control. If it was about security, they would have revoked the GMS licenses of pretty much every vendor outside Google themselves and maaaaybe Samsung, until vendors actually started caring about security. If it was about security, there would not be as many scam apps in the Play Store itself.
Back to your sister, the proper solution is to educate her (and everyone else) not to give apps unfettered access when they ask you to, plus let Google implement more security measures that systems like GrapheneOS already have (contact scopes, sensor permissions, network access permissions, etc.).