How to stop family sharing

Here is your guide:

Leave Family Sharing - Apple Support

Note that if children - generally <13 yo - are involved, there are some limitations.

In short: A child MUST remain part of a family; they can not be simply “abandoned.”

Moving a child to another family is a sometimes error-prone process requiring the 3x parties involved to carefully follow these instructions EXACTLY, and in the order written:

Move a child to another group using Family Sharing - Apple Support