Mother in law being nasty

We are a blended family, so I don't know if I'm overthinking this or not, and would love some advice.
My son with my ex husband is 17 and I think he has ASD.
He can make funny faces sometimes when hes talking.
My partner and I have a 6yr old son together, and he's started pulling funny faces.
My mother in law saw him doing it one day and said nastily, 'Ugh, you look like *my son* when you do that!'.
She's now said this numerous times.
I'm pretty offended she would say this about my son (who isnt her grandchild), but I don't know what to say when she says this?
I'm very much a people pleaser/pushover so I don't know how to respond to this.
Thanks