The site Owain posted is great, once you have set it up and you will ned to be in Bulgaria to do this or your sister as the account on epay has to be registered with a BG bank account, its all quite tricky to set up, but basically, you register on the epay site, and then you will get a reference emailed to you, which you take to the bank where you want to set up payments from.
You put in your atm card and punch in the ref and it is then registered.
Keep the bank account topped up and then (if all registered with Epay correctly
you will be notified that there is a bill waiting to be paid.
You go to epay online and pay it, it is that simple...once set up.
The red card just has the account number on it, but if you have an old bill about, you can use the ITN number (starting 292 though this could vary from region to region ) directly underneath your name/company name or the cl. number which is a 10 digit number starting 1001 (again this might be regional too)
You can also register with this EVN site to check if a bill is due but once your set up properly with Epay, you wont need this you will need your ITN for this though
http://www.evn-ec.bg/bg/haushaltskunden ... scheck.asp
I just checked my account on Epay and its in English mixed with BG cant see german, maybe try it in Google translate??