Sounds like she jealous of the niece but hey some people never grow up
Honestly the way you describe her it could be any reason no grown as* woman in her right mind would act that way towards young adults who are our future... Treating people with respect has no age minimum or limit... That lady needs a reality check and I'm sure she's going to get it soon...
Hope her daughters don't listen to her either
Perhaps she is not very self aware.
We used to call those women "church bitties" because they are attracted to churches. They are a toxic organism, like Ebola. Very hard to eliminate. Best thing to do is to stay away.
I've seen it many times in my life. If you can find it, read "Evil Genes" by Barbara Oakley. It might help you to understand her better.
Maybe or maybe not. Maybe at the moment, but not later.