Transcript for 'She was religious in putting on her seat belt:' Princess Diana's sister speaks out
We swish genevas and turn to a new documentary about princess Diana where we hear from her older sister and the question she still has about the night Diana died 20 years ago. Adrienne Bankert here with that story. Good morning, Adrienne. Reporter: Good morning. It's not often we hear from Diana's sister but she and others in the family are commemorating her death speaking candidly about their grief from 20 years ago. Often avoiding the spotlight princess Diana's eldest sister lady Sara shares openly about her torment the night of her crash. These two hours the presenters on every news channel were saying injured but expected to make a full recovery and I have no idea why but it made me so angry. Reporter: More from that rare interview and others in the royal family in the but bbc documentary "Diana: Seven days." She was religious in putting on her seat belt. Why didn't she put it on that night? I'll never know. It was definitely somebody she would go to if she was struggling. Diana's family have never, very rarely spoking before about her but I think for the 20th anniversary they were very keen to in the knowledge that lots of people will be coming out and talking about their sister, they wanted to have authoritative voices and wanted the right kind of narrative. Reporter: It features her equally famous sons who wonder every day what it would be like having her years and the duke of Cambridge. We go looking for her to talk to her, to play, to do whatever. She'd be crying and when that was the case, it was to do with press. The damage for me was being a little boy age 8, 9, 10, whatever it was, wanting to project your mother. They've often paid tribute to her work and legacy but for them to speak about how they felt about losing their mother was incredibly moving and it's not something that we've ever heard before. It was an intimate look at what they were going through. Diana certainly would have been proud of both of them as they carry on her philanthropy and graciousness WHE graciousness. The anniversary of her death is this Thursday, August 31st. I looked back on some of the research. Her funeral was -- is still one of the most highly watched events in television ever, billions of people around the world loved her and tuned in that day. The sons really wanted to make sure she was remembered and honored 20 years later. Well, it certainly is moving to hear their thoughts and feelings about their mother. That they still, of course, think of her every single day. Absolutely and I think still people are in touch with who she is. I have friends traveling to London for this event. They have been doing such good work in her memory. Absolutely. Adrienne, thanks very much.
This transcript has been automatically generated and may not be 100% accurate.