The simplest way of putting it is that you will be financially responsible for everything your sister is responsible for but without the benefit of actually living there.

As someone else already said, if you ever have to sign documents attesting to your financial obligations/debts, this counts.

You say you aren't worried about your sister not paying. I mean it's not a problem until it's a problem.

Many people cosign for others every day. Some people have a smooth experience and others get screwed ponying up the money for someone else.