After offering a kidney to her sister they discover they were both adopted. They go on a search for family and a kidney based only on their Mum’s old diary.PS Adoptive parents are dead, can’t see how to fit that in.

Switch it. A dead Kidney donor is found to be an exact match for her recipient, meaning they were twins. The recipient must go on a detective hunt to find out how and why they were separated, and the search leads her across continents and through some of the twentieth century's most tragic history.

This is an anti climax she fights for her life for the most part of the story then at the end decides to stop and die. It is sad and a letdown, if you want the MC to die then best to make it an act of sacrifice to help someone else. Your line:

I don?t want to struggle around people who think they obligated. I want to go home, please, I just want to die as your sister?.

Sounds melodramatic as it is a dramatic choice with seemingly no believable motivation except verbal justification via exposition.

My reaction is the same as TabooCouple's: bummer.