Old family photos - ownership

1. Scans of old photographs are not suitable quality for Stock. 😞

2. Anything published before 1923 is in the Public Domain.

3. Adobe Stock doesn't accept images that are in Public Domain.

• https://helpx.adobe.com/stock/contributor/help/submission-guidelines.html

4. Whatever you submit to Stock must be 100% your own work or accompanied by signed property releases by all persons or entities who can claim copyright.

• https://helpx.adobe.com/stock/contributor/help/ip-guidelines.html

The copyright ownership would have to be legally transferred from the original owner (the photographer) to someone else. You can't do that for images from your grandfather until those images enter the public domain, which is a murky area. Here's a webpage that describes when an image becomes public domain:
Public Domain Images—Are You Legally Compliant? - Sidebar Saturdays

Assignment/Transfer of Copyright Ownership (FAQ) | U.S. Copyright Office

Not sure all this legal effort is worthwhile unless you believe the images have commercial appeal.