I have been trying for some time to track down a book I read in the UK 20-30 years ago - maybe in the school library. It was set in France during the Second World War, with a strand in the present day. The contemporary thread involved a girl going to stay in the French countryside with her father who was working there. There may have been a ghost element to it. The 1940s story involved siblings, a girl of about five (Lucie?) and her older brother aged about 10. They were Jewish and were left by their parents in the care of local residents. They were hidden in a locked tower and then abandoned. There was a really harrowing description of them slowly starving to death, and the boy looking after his sister - there is a white rose in a glass and I think the girl dies whilst holding it, the boy dies a few hours later. The girl in the other strand uncovered what happened to them.
I have looked through many lists of holocaust/WW2 fiction and never managed to locate it.