“If youre not hiding anything youll let me look through this” is a gross manipulation tactic
You have every right to keep your stuff private, it is natural to not want people, no matter how close they are, going through your phone/papers/conversations/pictures.
Dont let them trick you into feeling bad because youre a human who wants privacy.
And social media! You can have a social media account that you don’t want your family going onto. Privacy is important.