MIL makes comments on my cousins 6 month old

My cousins baby boy is 6 months, just found out he has to wear a helmet for a while due to favoring laying on one side. My MIL has commented SEVERAL times to me about how large his head is… she is friends with my cousin on Facebook so sees the photos she posts of the baby. He is the cutest thing and no one else has ever made comments like that. If they feel the need… keep it to yourself!!!! So rude to comment like that about a baby IMO. If she says it again I think I’m going to absolutely go off. I’m not sure how I haven’t by now. I’m also currently pregnant and paranoid she will make some idiotic comment about the appearance of my baby. Needing advice on how to handle this type of situation without sounding like an AH.