So BIL is the flake. :D
The not eating thing wouldn't have bothered me. If she was rude, didn't at least try to chat with you/family, then yes, it is rude.
For me, if people assumed I disliked their food, or that I was insulting them if I didn't eat, i would be terribly hurt- I have IBS and anxiety, which in some settings, it is hard for me to eat, even though I want to. It is also hard to share these things with people, so I do try to eat a little but often times I don't. I love my family/friends, but sometimes it is easier on me not to eat.
I have a few people in my life like this and my brother's MIL is a bit like this. I call it socially awkward or she has control issues. Brother's MIL is very into 'dont want to bother anyone or put them out" which ends up having the other people involved either standing around all awkward or quite bothered depending on evennts and personalities.
We had a close family friend growing up who had to be in complete control of everything and anything. She hosted thanksgiving every year and although in chiildhood the memories were great with all the kids, as I got older I realized how controlling she was about everything. All get togeethers had to be at her house, even up until a few years ago her grown children had to obey this. When they all went places, they
ALL arrived at the same time and LEFt at the SAME time. Even my wedding that two of her married daughters attended who had been married over a decade. When her youngest met my BIL and dated, he had stories. He then married her and the drama with that wedding is something for bridezilla or motherzilla.
when you mention that she rarely eats at her own house so you can see it, I kinda wonder if she gets gas or some other embarassing digestive problem that she doesn't want to have crop up in front of others. Did she smile and say hello to everyone when she arrived and when she left?
It has been a few day, so I am in a more charitable mood. She was friendly and did chat with others. It was mostly the food thing I found odd, and I sense my mom was a little put out by 2 adults refusing to eat anything at dinner time