Hi, John. It is possible to use chemicals to bleach wood but I certainly would NOT encourage you to do this in place. The chemicals can be pretty trecherous. I think that the place to start is by giving the wood a good cleaning. Mix up some Dirtex cleaner with water according to label directions. Wash the walls with a damp sponge at several locations to see how much and what is coming off. Rinse with clear water on another sponge. You don't want to get any more water on the walls than necessary. Start with just some test sections at a few different locations to see what kind of results you get. Dirtex is available at most specialty paint stores.
I can't tell you what you will see doing this but it should give you a lot better idea of what the siding looks like. My guess is that you have a heavy buildup of smoking, fireplace, cooking oils and other residues. The cleaning should get a lot of this off. If the test sections show promise, you can continue cleaning. If the walls and ceiling are still too dark, I think that you should consider painting. If you do decide for the painting, the first step will be to wash the walls with the Dirtex anyway.