First, I am sorry for your loss.
I love the show Antiques Roadshow and this past season someone brought in a bunch of valentines and other holiday greeting cards, like for halloween and such. Some of them are VERY valuable. You should have them appraised by an expert or at least maybe your local museum could tell you the proper way to preserve them. I do know that laminating them or glueing them to a foundation (ala scrapbooking) dramatically decreases their value.