Who says such a thing?

Ok so yesterday after church we was eating lunch and my son is 1 year old now. My son likes for some reason to take his hands and hit his head I think he picked it up from daycare 🤷‍♀️ He’s always moving his hands and playing with stuff..He’s perfectly normal though..Well my sister made the remarks “what does he have Down syndrome or something? Because the baby was playing with his hand and making faces..I mean who does that ? I didn’t say anything but my mom heard her and it really upset us both. How would you respond too this ? I mean I think my son is adorable and thankful he he’s healthy too!