Should I charge her?

My sister in law asked me to sell her my formula the one that my son doesn’t use. She doesn’t qualify to get WIC program and I do. I switched my sons formula so I have the ones he left. Which are the ones she uses. I also have to pay from my pocket when I ran out of my sons formula because they give me 4 cans so that’s not enough for him. And yes I do pay full price when I have to buy them. I was thinking of selling them to her but for a cheaper price half the price. Idk if it’s a bad thing since she’s my sister in law that I’m thinking of charging her?