If your grandpa died and you lived with him and your grandma.... How do you even know what to say to help them feel better?
Some times its best to be her shoulder to lean on, make sure you tell her you love her every day, give lots of hugs and kisses and let her know you are there if she needs you, and do extra things around the house.
Sometimes a hug and silence mean more than words.
Don't say anything.... just be there. :/