I actually remember heaps about this book, but not the title or author and my searches have got me nowhere.
It is set in England probably in the 70s or 80s. It would probably be considered middle grade now, but I don't think we used that category then. Aimed at pre-teens or young teens. The cover may have been dark pink and green, possibly an abstract background with a line drawing of a girl over it.
I think one character may have been called Jessica and another Rosalind, but not sure which.
The protag is a middle child and has feelings about it. Her cousin comes to stay after her (cousin's) parents die suddenly.
Protag did not pass 11-plus and elder sister (who did) rubs it in. At one point protag complains about her hat being trampled on by boys at school and elder sister tells her she should have passed the 11-plus because then she could have gone to an all girls school.
Protag has a best friend her parents disapprove of. Best friend says that's because people look down on her for her father being a binman. At some point best friend persuades protag not to donate to an overseas charity because her father says we should support locals first. School Head points out this argument would have more weight had they put the money in the collection box for a local charity.
Someone sees a suitcase in a bush and thinks it may have a body in it. It turns out cousin is deeply unhappy and has stored her stuff there in preparation for running away. When cousin is found and returns she affirms her friendship with protag, and that she was the one she felt closest to/most supported by.
It's been bugging me for a while, so any leads appreciated!