How can I ensure that my iCloud is safe and is not shared with anybody including family? Can I opt out sort of speak

I understand that when somebody is trying to invite you to family sharing you need to except an invite but when you’re under the same household it’s not very hard for someone to pick up the phone and hit except without your knowledge. How can I assure that my iCloud is safe and it’s not shared without my permission is there an opt out or some thing to secure my information?