Hi PyneappleDays,
The way I see it is that there will always be outside enablers where it comes to our pwBPD. My dd19 has become a master of spinning a good yarn since she was 3yo, so I have had many encounters with many people over the years who took her side of the story and accused me of being a bad mother. some of these very people were members of my own family or my very close friends and... .Yep they all believed every word dd told them and they all ended up getting burnt by her too
There is a saying that birds of a feather stick together which may be the case with the family your dd is living with.I know that it is hard to watch, but your dd is 19yo now. She will make mistakes and will even continue to make mistakes whikle she lives with these people.You dont have to like how she is living her life, but just be there without any accusations or I told you so's because in my experience they dont work. If anything it will just drive your dd;s behaviour further underground.
What has helped me is reaching Radical Acceptance with my dd. I still dont like many of her behaviours but I have learned to accept that is the way she is. It hasnt happened overnight I can tell you that but it has hepled me to have an easier life. I kinow that I cant change my dd. She would have to change herself... .and little by little I have begun to see these changes... .And wherever possible i stay away from lecturing my dd. Even though you are right in your concern for your neice driving in terrible weather conditions, your dd has only seen it as an attack or that you are trying to prevent her from staying over at her aunts.And of course because she has interpreted it this way she has relayed the message in the same way. You must remember that our pwBPD are very immature in their thinking... .so all they ofter hear is the word "No"!
I think in future it may be better to speak to her aunt or whomever concerned directly expressing your concerns. If it were me I would have expressed my concerns and then suggested another way she could get to her aunts safely... .maybe by taxi perhaps or bus if possible and offering to pay if neccessary.That way it solves the crosswires.
Last Year my dd had a meltdown over the xmas holidays while she was here... .so this year she will only be coming over for the day itself... It was dd's idea and I think it is a good one.

It takes the pressure of her to be jolly and festive and it takes it off us being on tenderhooks all the time either trying to involve her or not to upset her... .If she decides to stay longer then thats ok too, but this way I think she finds that there is less pressure or expectations.
so yeah we have all learnt from last years events and we are all hoping for a more peaceful and more enjoyble one this year.
all the best jsf