How do you explain to your mother-in-law that it's not okay to put a b…

How do you explain to your mother-in-law that it's not okay to put a baby in a seat before they can sit themselves? She puts her 5-month-old in a seat, and she's told her many times, but she says nothing will happen, just like all of her other children were put in seats before they could sit themselves. She has already explained this to her, and she said, "Look at Dr. Kamorov's." But she doesn't understand.