Fixing my credit or hers?

So my sister and I moved into a house last year and the electric bill is under my name. I would give her my side of the payment and she would have her part. Well a few months down the road I started getting letters saying I was over due and owed about $600 and I was pissed bc I've been paying my part but apparently my sister never paid it. Well I ended up switching the bank account info to mine after wards to fix my credit. I've been relying on the electric bill to slowly help fix my credit and today my sister asked me to switch it to her name because she has bad credit. But I'm still trying to fix mine. I really don't want to switch it over to her name. I'm wondering if she should just go talk to her bank about opening a credit card (she has landmark credit union). What do you ladies think I should do???