A long time ago I was desperate for the love of a boy and I never saw how flawed he was. In order to please him and his family I let his siblings treat my siblings very poorly.
It went against the sibling code:
"You mess with one of us you mess with all of us."
To this day I regret that. If you want to really know someone watch how they treat your family and how let their family treat your family.
I will not say that my brother and sister are perfect. Oh, they drive me NUTS! And I will tell you all about it if you let me. The thing is they are mine. You say nasty stuff about them and we are gonna have an issue. You treat them poorly then you better not intend to be my friend for long.
My sis has lived through shit that no one should have to deal with. The bullying she was subjected to in school was stuff that would bring assault charges in today's climate. It has left scars and so, maybe, she feels slights a bit keener than others might; add that she is pregnant to that and you could say she is a bit tender hearted these days. Understandable.
As my kids get older I want them to have an even better relationship than my siblings and I. How can I expect that if they are allowed to see me sit by while someone treats their aunt like she is an outsider not to be included in the group? So, just know that if you treat anyone in my family like crap without very good cause (and some personal issue you have going on is not good enough) than I am going to call you on it and make sure that it doesn't continue.