As best I can recall, I read this novel in the late 1980s, when I would collect lists of recommended novels for high school students planning to study the humanities; I may have read it a few years later. So the novel was most likely published in the mid to late 1980s; from what details I remember of the novel, it could have been written any time in the 20th century, though stylistically it seemed at least post-WW2.
It was set in a rural area, and I think in the UK or Ireland. The protagonist was a young woman, who was exceedingly shy, and who was bullied a bit by her sisters, who did not respect her. They may have been step-sisters; I remember that I was reminded of Cinderella. At one point the protagonist missed a spot while washing dishes; her sisters complained a great deal about it, one saying that she should always use a mop.
The protagonist is somehow introduced to a young man, a bit younger than her, who is sick, and who spends almost all his time in his room, playing out detailed fantasies with toy soldiers. They seem to be soul mates. He soon dies, and the protagonist manages to collect his clothes and toy soldiers, recreates his room, and spends all her time there, remembering him. Her family tries to put a stop to this, but she finds the strength to push back, and so secures her own identity.