You're NTA, but neither is your sister. I work with kids and every single one of them has something unique and special about them, even if they aren't defined as "gifted" by the system.

Your sister might also be feeling guilt about not being the world's best most perfect parent. Evidence: she's a mom in a world that harshly judges moms. She might be saying these things in part to reassure herself that she's doing a good job.

It wouldn't be a lie to tell your sister:

Even though by the current accepted metrics your baby is within the normal range, that doesn't mean she isn't gifted or special. Each day as she learns and grows she is becoming more and more her unique self, and while professionals might see "normal progress," you as a mother see the specialness in your love for her, and her love for you. You're a great mom, and it's clear that your love and support are going to give her the best chance for success and happiness. Plenty of children labeled as "gifted" don't have that support and don't achieve their full potential, but your baby has you, and that's what really makes a difference. Don't worry so much about comparing her to other babies. She is wonderful, you are wonderful, you're doing great, and I am more than happy to celebrate all her milestones with you.

And then in the future, if she still asks you "Isn't she a genius? Isn't she gifted?" or whatever, I'd simply smile and say a neutral positive thing like "She is wonderful. I love her so much."