What would you do?

Today I met up with my family at a pumpkin patch my brother went up and kissed my daughters and when I got a look at him I saw he had a cold sore, I asked him if he did he said I don't know maybe and he knowing this kissed my kids I got so upset with him how could he be so selfish, my cousin and mom thought I was exaggerating by leaving right there and then, what would you do?