Wow ~ ya I would be reluctant to sign on the SIBLING at this rate and committing to another few years of having to deal with that!
I would definitely NOT be giving the sibling a DEAL considering they are a PITA ... if you are considering taking on the sibling than I would be doing new contracts for BOTH children at the NEW FEE and having that contract CLEARLY state that immediate termination and forfieting of security deposit will occur if there is ever a payment issue in future and that the code of conduct for behaviour, communication and conflict resolution's must be followed at all times ~ no way would a client throw money at me or slam my door or 'ignore me' in my own home and hope to continue in my program without a commitment that it would NEVER happen again!