Right now, the community is supported in English, I hope you don't mind me replying like this.
If you're paying for the other accounts directly with your creditcard, and they're already part of your Family plan, you don't have to cancel them. They won't be charged for Premium individually moving forward.
In case the accounts are set up through a third party payment, you'll need to cancel them first, so they can join the plan.
To cover all bases, make sure your family members joined your plan with the same accounts, instead of creating new ones.
All the best.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp