Girls, hello everyone who finished GVS, I plan to breastfeed for 1.5 y…

Girls, hello everyone who finished GVS, I plan to breastfeed for 1.5 years but my mother-in-law says we need to stretch it to 3 years. She says this is mother's milk, it's always good and now we don't understand what we're feeding them. How do you convince her that breastfeeding until 3 years old isn't necessary?