Can I Get My Sister's Kids Vaccinated Behind Her Back?

GOOD QUESTIONI am the primary caregiver for my elderly mother, who has lupus and thus a compromised immune system. My sister has four young children, none of whom she has had vaccinated out of fear that vaccines cause autism.

My mother and I watch all four children regularly but have become increasingly uneasy in the face of outbreaks of measles and other communicable diseases. My mother and other relatives have implored my sister to reconsider her anti-vaccination stance. We have told her that if she doesn't, we will have to stop watching her children, which would be a significant hardship for her. (My two older nephews were both denied entrance to a great local public school because my sister refuses to vaccinate, leading to even more babysitting time at my mother's house.)

The health center where my mother receives regular care hosts a biannual, reduced-cost vaccination clinic. Would it be unethical for me to get the children inoculated there the next time such an event is held? I don't want to alienate my sister, but at what point does the common good outweigh individual choice?