Why do my parents hate me :'(?

Its my birthday next week yet my mum turned around and told me she forgot about it so she hasn't ordered my cake, then she tells me im not getting anything for my birthday so not to expect anything but then she goes and spends 200 pound on my older sister coz she wanted new clothes. They always get her whatever she wants and they never buy me anything, in fact that hardly even pay attention to me but i cant believe they aren't getting me anything for my birthday especially when their spending money on my sister for no reason and it hurts even more that they forgot it was even my birthday :( why do they hate me so much?