hey,
sorry to say it, but it doesnt seem right. unless she needs that help and they are trying to protect you by not telling you?? i get Disability allowence as do many of my friends, but a lot of them have to exagerate how much help they need in order to get the maximum amount. they all need it most of the time but they judge you on whether you can be left on your own or not which is total crap.
if your sister is younger than you then that might be why, if shes like under 12 i think you get more money for less of a disability depending on whereabouts you live and what your doctors like.
maybe you should talk to your parents and see how ill your sister really is, if she is in a unit then she is obviously not well and needs the money, but if shes much better and coming home soon, i dont think she needs that much unless its specific from an anorexia allowance. talking to your parents will probably be best to find out if shes okay. i hope she is and hope she gets better soon
hope ive helped,
PM me if i can help anymore or if you ever want someone to listen to you
Hannah X