I think we should bear in mind that having Vicky for a mother was very difficult for Wilhelm, since she set him standards which were quite impossible to achieve and never gave him credit for anything he did. Rohl quotes one rather sad episode when Wilhelm, aged about 19, sent his mother a photograph (inevitably showing him in uniform) and got a lengthy reply exhorting him to clean his teeth better. She was nothing like as demanding with the three younger girls, and they, inevitably, got on better with her.
It is a recognised phenomenon that parents tend to apply strict standards to first-borns, who bear the weight of expectations and ambitions, as well as rules. In the nineteenth and early twentieth centuries we can also see this with heirs apparent - look at the way Victoria and Albert treated the future Edward VII!
Ann