Childrens book about a twin whose lives in a house with a different family on each floor

[Reader]

read about 6-7 years ago. its about a girl and at first she speaks to her twin and stuff so the reader thinks she(the twin) is still alive but we later find out she actually died by drowning one day (in a ditch) and so the twin ends up moving schools and stuff. the family upstairs(they live in the basement) does some exchange program with some american family and the girl opens up to the boy(think his name is declan not sure) about her sister for the first time

[Rainbowheart]

Is the girl a teenager or a younger child?

Did this book take place in the UK?

[Reader]

teenager and yes its in the uk

[Reader]

also im not too sure abt if the sister drowned in a ditch or not. the two twins took a bus somewhere then twin2(one who died) finds a group of boys who she starts speaking to and goes with them to a forest? which is where she drowned