Hay, I totally get your worry po! A few of us mums experienced similar transition issues kasi talaga ang tummies ng babies ay sensitive sa formula changes, and it looks like ang 1-3 formula might be too rich or different for her digestive system right now. I'd really suggest bringing her to your pediatrician para ma-check if may lactose intolerance or if kailangan lang ng gradual transition (like mixing the two formulas for a week or two instead of switching cold turkey), kasi every baby's tummy ay iba-iba, and the doctor can give you the best plan for your little one. Your sister's concern about age is sweet, pero honestly ang health ng baby mo matters more than the age on the box, if she's thriving better on 6-12, that's perfectly okay din!