Joni, There are 2 easy finishing techniques for knotty pine that you might consider. The first is 2 coats of oil based polyurethane. The oil based poly has an amber look that I think is very pleasing. The second is to thin cheap, white paint way down and paint that on then wipe most of it off. You can control the look by how much of the white you wipe away. When this has dried, you can apply a coat or 2 of water based poly, which is clear. With either method, you will want to prep the wood as you would with any finishing project.
The sun is a bleaching agent. It will affect everything from fabrics to paint over time. The best preventive to color shifts because of this is to block the sun. You can do that with drapes or other window treatments. All that cloth around your windows did have an original need involved. It was to block the sun. If you have modern glass, that will help; but nothing will stop the bleaching from happening with direct exposure over years. The cheapest way out is probably really cheap LINED drapes, with better ones over or around those.