Some suggestions of gifts that you can get for your mom include an antique painting or a baked cake. You could get a customized bag or a gift certificate for your sister. Reading material such as cookbooks and medicine books would be appreciated by your grandmother.
It would depend on their interests and hobbies. I always have problems finding gifts for my Mother as she is in her 70's and has almost all she wants or needs. She just enjoys the family being together. I usually get things for her that she wouldn't spend the money on herself for. She likes to go out to eat, so I get her gift certificates to places I know she would like to try, but feels are too expensive. She also likes going to the Casino, so I'll make her a card and tell her that's what she must spend the money on. A treat for her. Just think of what they like but wouldn't buy for themselves.