When we lost my dad and my FIL, I made sure my mom and MIL got an 'its your anniversary" card on their anniversary....just to show that I remembered. They seemed to appreciate it, and I think they were touched. If I remember correctly, we signed it saying we knew it was a special day, and didn't want mom and dad's special day to go un-remembered, since if it wasn't for THAT event, we wouldn't be here!
Unfortunately, both mil and mom are now in the throes of Alzheimer's. MIL no longer even recognizes pictures of FIL.....so I am glad we remembered when we did.