Gift giving can be tricky. If your sisters said they want to opt out, do you really want to still give them something? Will that irritate them? Or will they find it really kind and sweet of you? Will they say something like, "We SAID," or "WE all AGREED not to do gifts?"
Sometimes people are really firm when it is decided not to do gifts, and they really prefer no gifts.
Sometimes people feel caught off guard as in, "Ohhhh, you got me something. I didn't get you anything," and that makes them upset.
You know your sisters best. I would stick with something small, homemade and edible. It doesn't have to be fancy. You could make some kind of quick bread (banana bread, pumpkin bread....) in mini loaf pans, wrap them up in clear plastic and tie on a pretty bow. That way if they don't want to eat it right away they can toss it in the freezer. (You could buy the bread in a ready made mix, or make it from scratch.)