Well half of those that do get this registering issue does not fill in the required fields correctly but there could really be a few other things.
First off she must make sure she actually does this, fill in the username and password make sure they accept the agreements by registering, writing the antispam code correctly. Not creating a username that someone already has.
If they do all this, the account should get created, and in that case we should be able to see if the username is awaiting activation.
If you got the username of your sisters account that she created, I would be able to look up if it even exists and activate it directly. But many of the times the account doesn't even exist and we can't do much about just that part.
Something they also can try is using another sort of email address. Lets say they've
[email protected]. Maybe they should try to register with another mail like
[email protected].
Also things like trying different usernames, and only stick to a-z can help as well.
The activation mail should get sent right away really.
But feel free to post the username of the account your sister created and we will be able to check up on it and activate it if it does in fact exist.