Overreacting ???

So atm I share rent with my mom … this matters because my sister who doesn’t live here sends her kids here on the weekends to be with their grandmother. Regular stuff.. well starting tomorrow her kids don’t have school/daycare for the week.. So all the of her kids are here 14, 8, and 3 y.o. for the week because she still has to work. When i seen my niece (8 y.o) first thing I said was what was wrong bc she looked uncomfortable. My sister just says “oh she ate too fast so her stomach hurts.” She seemed fine for a couple hours and then about 1.5 hrs ago she started crying and saying her stomach hurts. My mom left to get Pepto meanwhile my niece was just laying there with a red face crying and saying her stomach hurts. When my mom got back and gave it to her she threw up.. she continued to cry nonstop and clutching her stomach. We called my sister and all she said was that my niece is overreacting and not to take her to the DR. Well we cannot just go to sleep with her screaming in pain and my sister never offered to come get her child. So i called my sister and said that my moms taking her to the hospital.. which she got mad about but really what else could we do with a screaming child? Mind you my mom is very high risk so any sign of sickness myself and my child will go to extremes to stay away from her. Also now they expect me to watch the 3y.o. But i put my son to sleep early tonight so that I could get some rest for work tomorrow. They think i’m an asshole for not helping when I never ever agreed to any type of babysitting in the first place. This was between her and my mom. Wouldn’t you expect the parent of the child to pick the kid up and take her to the doctor? Mind you no one helps me with my son in any way and she would be livid if i sent him to her house sick. I think its so disrespectful to risk our health and to expect me to take over when I work tomorrow morning too. My niece is also crying so loud shes going to wake my son and we need to be up by 7am. I understand that this is a break from all her kids but she cannot just dump sick kids on other people. Please lmk if i’m being irrational and sorry for repeating a lot