If my sister wants to join Sylestia as well can we use the same computer? I know some websites like this use IPs to limit the number of accounts a computer can access to avoid cheating and whatnot.

I believe you can, but to be on the safe side, you might want to mail the admin, Krinadon, to inform him that your sister will be joining as well, as he can make an exception in the system for your accounts or something like that. =)