Adding second iCloud to Mac

Hi

I have a new Mac Laptop which I have set up with my own iCloud. My mum has an old desktop PC with lots of photos on her hard drive. She also has an iPhone with her own iCloud. I'd like to help my mum save her hard drive photos to her iCloud so she can see them on her iPhone. Her PC is too old to upload them to the iCloud from her PC, it crashes.

I can take the photos off her hard drive onto a USB drive, and then use this drive to upload them onto my Mac. But is it possible for me to use my Mac to upload the photos to her iCloud rather than mine? Can she log onto my Mac computer with her iCloud? I can't see how to do this.

If anyone could help me understand how to do this, I'd really appreciate it

Thank you

Noah