"You shall not cook a kid, etc.": This tells me only of its mother's milk. Whence do I derive (the same for) its older sister's milk? It follows a fortiori, viz.: If it is forbidden to cook it in the milk of its mother, who does not enter the shed alone with it to be tithed, how much more so is it forbidden in the milk of its sister, who does enter the shed along with it to be tithed!