Implement rules that if a parent has more than 1 child, all siblings have to sign and agree to a joint bank accounts between the parent and the chosen child. If other siblings are not aware of the joint bank account the account needs to be frozen and investigated to ensure an agreement was made.
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There have been instances where, after the passing of a parent, the child with joint ownership inherits everything in that shared account and does not share any with other siblings. I think joint accounts with parents need to be reviewed.