I have a redwood cabin that was build in the early 60s. Someone years ago stained one side of the cabin with an opaque redwood looking stain. It now looks worn and flat

I tried removing the stain by lightly pressure washing but the wood is very soft and I did not want to damage the wood.

What can I use to remove the old stain and then what can I use to restore the previously finished side and protect the unfinished sides?

Stains are penetrating, meaning that they are absorbed into the grain of the wood. The only way to remove the stain is to sand the planks down to the point where the stain stopped being absorbed. That's a lot of hard work and not worth the effort. The options are to clean the existing planks and restain using the same or darker color or replacing the planks and starting anew. If you use a completely opaque stain then you might be able to use a lighter color on the previously stained portion but check with the stain manufacturer first to make sure that the product is being used according to the application guidelines.