Are you depressed because you see your sister suffering now? It is impossible to replace someone exactly, so my advice would be not to try and find someone else who could visit.
Have you tried taking your sister out and finding some mutual pastime you can both enjoy together?
In my personal experience, I find it hard to tackle the feelings of depression head on, if you can get around them even slightly you may find that tackling them from another direction works for you too.
Good luck, hope you can make use of this.