From what I've read, the family went to America to attempt to influence the powers that be (ie Donald) to waive the diplomatic immunity that allowed this woman to leave the UK, or to find ways of ensuring she returned to the UK to allow justice. She had given her commitment to the investigating police team, that she would stay until their investigation was complete, then left the country.
It's clear that Donald Trump had arranged a photo opportunity. He had AS in a separate room, asked the family to meet her, they saw a rank of photographers and very wisely, imo, declined the offer.
This was as ever, all about Donald and in no way about meeting any of the needs of this family. Their position has always been that she should return to the UK.
How on earth is it acceptable for anyone to leave the country in this way? If she was a spy, I could understand (almost) the need to keep her identity secret. She isn't. I loathe the way she's described as "the wife of a diplomat", as though that's all she is in the world. However, it does seem that being "the wife of a diplomat" enables you to avoid the judicial process when you cause an accident that leads to the death of a 19 year old man. Shocking and unacceptable.