Come on, Buddy, just open your heart and say it!
My serious answer is that you seem to have a problem expressing affectionate emotions to anyone who isn't one of your kids. It makes me stroke my invisible beard while saying, "Well that's interesting," in an Austrian accent, but I don't think that means a lack of love for your mom and sister. There are just people more comfortable returning those words than others. For me, it's awkward, but I don't see a cost in indulging someone I genuinely love. It can be selfish and insecure to try and prompt an "I love you" out of family(and can be abused), but that's also very human. Those words can make a person feel good for a little while, especially if they're at an age where they're contemplating their mortality. There could be a bunch of other reasons why your mom wants to hear it more often, and you could ask her why.