It's hard to say without specifics because everyone behaves like brats "at times" so it really depends how often and how much. In a family people can disagree about how much they're entitled to and sometimes they do need to be reminded what the limits and boundaries are.
They can be lovely. My husband can be fine. As can my mother. But sometimes they don’t act as I would wish adults would.
Well, no-one is perfect and so long as they are lovely and fine towards you most of the time it is probably OK. And people can't always behave like adults, especially not in the family.
I am sometimes embarrassed by their behaviour and sometimes just sets my nerves off.
That could be a social anxiety thing(?) They are adults, they are responsible for their own behaviour, it doesn't reflect on you.
I try to not let it show but although I often speak frankly and calmly ( I was a secondary school teacher so well versed in all that) I was wondering if anyone had any bright ideas.
Have you ever looked into transactional analysis? It's all about shifting effectively between parent/child/adult ways of communicating. I know people who use it to manage communication at work without alienating people , as well as with family relationships.