By 'cool' Aunty I don't mean fun etc. I mean make the dc question our parental authority
Like what? I get accused of this also. What's really happening is my sis influencing and manipulating her dc into treating me and seeing me the way SHE does. Si then, naturally when you correct the dc, this gets twisted into 'undermining the parents authority" 
Examples:
"Your aunt's life is a failure/big mess because she doesn't do x/y/z"
My response, "WE choose how to live our lives and if WE are happy with it then that;s all that matters"
"you aunt is 'haram' and will not be allowed in heaven, and cos she doesn't follow the koran anything she says or does is wrong"
my response is to share my own beliefs and values, and she HATES this because she doesn't want her dc to know that you can live a happy, successful life without being muslim.
"you HAVE to go uni, get a degree, a professional job, married, mortgage and kids - in that order. If you do things differently then you will fail and be a loser in life"
My response - you don't HAVE to do any of these things if YOU don't want to. So long as you are happy and content and pay your own way in life - THAT is what matters.
"your aunt has bf's instead of getting married, she behaves like a prositute"
Well, this one made me see red. So yes, i told my nieces how their mother did the same thing, had bf all through college and uni, had sex outside of marriage and lived 'in sin' with their father for 5 years before getting married. i brought out all of her hypocrisy for her dc to see and judge for themselves.
Now they know the real truth they can judge for themself - and that is what people like you hate.