What happens with access to photos of family who share my iCloud?

That isn't something I have much experience with. Post the question in the Photos forum: Photos for Mac - Apple Community

You will likely get a response from léonie who is very knowledgeable (as are several others there).

Her photos will only be synced to iCloud if she turns on iCloud Photos in her iPad and/or iMac. If she turns iCloud Photos on in both devices, they will also sync to the other device (i.e., all of her photos will be on iCloud and on both devices). She can reduce the storage needed on a device by turning on Optimize Storage on the device. See information in: Set up and use iCloud Photos - Apple Support

She will continue to use her Apple ID for iCloud. If she wants to access your photos, she could do that by using an internet browser (e.g., Safari) and logging into www.icloud.com using your Apple ID and your password -- but she could do that whether or not you are sharing an iCloud storage plan.

More information on sharing an iCloud storage plan is in: Share an iCloud storage plan with your family - Apple Support