system photo library on an old mac

My wife hadn't used our ancient iMac for ages and regularly used iPhone / iPad with Photos, and I've recently cleared it for recycling. I'm now setting up a new account for her on an old shared MacBook Pro. Miscellaneous old files were copied across from the old iMac via an intermediate drive but no attempt was made to do a full sync.

On the Macbok Pro. signing into her iCloud account and opening Photos a message complains that features are only available in the System Photo Library - which I'm guessing was on the old (now cleared) iMac. There doesn't seem to be any way to identify where the System Photo Library lives as far as iCloud is concerned.

Is it safe to create a new System Photo Library and allow a merge between her extensive iCloud Photos and what is essentially an empty library? I truly don't want to mess up her Photo library?

Thanks,

Antony