I've never had someone say that to me. I've never had someone care so much about my hair care that they'd feel the need...unless they're suggesting a product I might want to try out.
Each person has different grooming and personal habits. I wouldn't knock someone who enjoyed focusing on polishing their nails, skin care, clothing, physical fitness, makeup...or how much effort they like to put into their hair. After all, I have seen people with straight hair still spend time blow drying their hair each day to give it body and then spritzing it with hairspray so they get lift at the roots. Putting some products in to tame my curls and then diffusing my hair doesn't seem like a big deal to me in comparison.
I wouldn't sweat it. Your sister will get used to any routine you start up. Once the novelty wears off I imagine she'll stop commenting. Siblings always like to pipe up with useless comments from time to time. It's kind of their job to be annoying...it keep us on our toes.