The file size is fine, it can be up to 8 megabytes per image. I'm not sure why it is failing, but such failures are usually due to a problem with the server itself.
Could you maybe get in touch with your sister's server admin?
I see she's on a very large server where the admins don't respond to personal messages, it might be easier if she signed up on a different server? It will probably work straight away, and a smaller server would have accessible admins.