girl finds her mums old diary where she writes that she killed her sister (the girls aunt) but she actually got hit by a car

This is a book I listened to as an audiobook on my mp3 player in the early 2010s. I believe it was set in the UK or Ireland.

A young girl (aged like 9-11) has an aunt who died, but the whole family never talks about what happened. and one day the girl finds her mums old diary, where she wrote stuff like 'I killed my sister, this is all my fault'. so the girl assumes her mother is a murderer. so she confronts her but then she tells her what happened.

They were at a girl scout/girl guides meeting and the mother was mad her sister had to be there, cause she enjoyed having time away from her. she made fun of her sister with her friend and I think pulled her hair. the sister ran out crying and the mother ran after her. but the sister had ran into the road and got hit by a car and died. And the mother always blamed herself which is why she wrote that stuff in her diary.

I need to know what book it was, please help reddit