Joint holding demat account can withdraw their rights without consent of joint holder..??

Hi, We have joint holding demat account since 2011 in name of A ,B & C.whereas my father having these shares in inheritance from his father & but due to his age ,he cant do sign properly,so he had opened account in name of his wife,Son1 & Son2. Right now demat account is already in dormat form,Mr.B is not singing on KYC form & KYC documents to broker.Mr.A & Mr.C are ready for every thing for giving KYC form for singing & also ready to give KYC Docs. But broker insist that if all joint holders are ready for singing KYC & giving KYC Documents then & then only you can operate joint account. My Questions :: [1] Is that possible that in joint holding of demat account having same rights of all holders then ready person Mr.A & Mr.B can take equally shares from demat account [out of Mr.A & Mr,B,Mr.A is a Senior citizen]..?? if yes the please refer strong evidence for the same refering law or any section of act. [2] What are the other ways to come out from this ambiguity ..?? please suggest