I'm so freaking frustrated with my sister right now... We had a family get together at my mom's…

I'm so freaking frustrated with my sister right now... We had a family get together at my mom's house on Saturday and she called me last night saying that she took her son to the doctor and found out that he's got hand, foot and mouth disease (which is extremely contagious)... Her son was obviously playing with my kids, which puts mine at risk... But, I'm mostly worried about the baby because older kids are less susceptible to it..... It's just bullshit because she knew he had blisters in his mouth... She shouldn't have brought the kids at all...