Photostream invitation won't accept

I am trying to invite my sister to my photo stream and I am trying to join her photo stream but for some reason, our invitations just aren't working.

We each receive the invitation, but it then says 'invitation revoked by user' or words to that effect.

My sister had the same problem with my father's invitation and I did not so I am assuming the problem lies with her phone.

Can anyone suggest any troubleshooting tips here?

All our devices are on iOS 6 here btw.

Best regards

Felix