Hello! I have an iCloud storage plan for 200Gb an a family share and and at the month end I have to pay and olso the other member of my family share is paying?

Read What is Family Sharing? - Apple Support to understand how a Family Sharing group works. The point of it is for members of a group to share the purchases of all the members.

One person pays for the iCloud+ storage plan subscription, and all the other members benefit by being able to access the space acquired by the subscription.

Apple summarises this at the head of the page: One adult in your household — the family organizer — invites your family members. When family members join, they get instant access to the group’s subscriptions and content that are eligible for sharing. Each family member uses their own account, so everyone’s experience stays personalized and private.