I have received a message saying:

You have received an Official Warning for the ownership of multiple accounts. Each individual user is permitted ONE account and may not create additional accounts for any reason, regardless of circumstances. Any additional accounts you own have been frozen. Should you create more, all of your accounts will be permanently frozen and you will not be allowed to return.

I'm playing on the site with my sister..... where can I report this? :O

go to Help and file a Ticket

thanks!

you are welcome!

It'll be useful to give your sisters username and in what ways the internet is shared, like same house hold or same computer. Also to limit any gifts and such. Likely your sisters account was made after yours and is frozen now? If so have her make a ticket from her account to also talk to staff.