Mil just rang me. She has a history of anxiety and has had treatment for this. She rang to say that she had just rung Dh's 65 year old aunt, who lives near us, since there is measles in our area and she was worried that Auntie C, who never married and has no children, might catch it. She had left no message as she didn't want to worry her. I explained that DH and I have both had it and both kids had MMR, so there wasn't really anything I could do to lessen the risk for Auntie C. I asked what it was that she wanted me to do, exactly? Auntie C is retired, plays hockey at seniors county level, plays in two orchestras and recently trained as a warden for her local nature reserve. She has a PhD in Biology and used to teach it at University level, so really, I don't think I can really tell her to stay at home in case she catches something! I left her a message asking her to ring her sister and reassure her that there were no spots! However, that's not normal on the part of Mil, is it? DH will dismiss it as just his mum being his mum but I wonder if there is more to it.
It doesn't really sound OTT, she has heard of an outbreak of something in the area her sister lives in, she is worried that her sister might catch it and called you and discussed her fear. She just sounds like a caring sister to me.