CCM cannot merge accounts. If there are two family accounts associated with one family in your system, below is how to resolve this issue: 

Cancel the registration(s) in the not preferred account and re-register them in the preferred account. Please refer to Resolving Mistaken Duplicate Registration for Same Customer

Note: If funds are associated with canceled registrations, refund the money instead of moving it to Account Credit Balance. Account Credits are not transferable to other accounts.