Fight with my sister

So my sister asked if I could come over and help with her 6month old while she does laundry and stuff like that, I have watched many children in my lifetime and I work as a activity assistant in a daycare. Well earlyer I was dressing the baby and she started having a fit because she didn't want clothes on after a bath and my sister came over and says "I dont think you know how to dress her" like wtf should I be mad?