Hi, I share a family account and was wondering if I backup to the shared iCloud, will everyone in the family have access to all my stuff

No, (unless they know your password) not even if you are sharing an iCloud storage plan. See the following from Share an iCloud storage plan with your family - Apple Support

When you share a storage plan, your photos and documents stay private and everyone keeps using their own accounts with iCloud—just like when you have your own storage plan. You simply share the space available in the iCloud storage plan with your family members, so you only have one plan to manage.

No such thing as a shared family account. Apple IDs are for individual use only.

Each person should have their own account / Apple ID.

As such, when sharing iCloud storage each person's files and data are kept separate. And nobody can access other persons stuff.

You are just sharing storage space, not access to your stuff.