I used my sisters toothbrush
Using your sister's toothbrush is disgusting. Is there a reason?
She took mine and used it, revenge.
Maybe she doesn't even know she is using your toothbrush and might just be an accident. Even if she used your toothbrush doesn't mean you should use hers. It's not only gross but it's not right to use her belongings without permission.
It's fine, I'll just get another toothbrush.
That sounds good to replace both of them. Maybe you can get different color ones so there wont be any mix ups.