Usually, my brother has to sign my sister's hand when he goes out, or are couples like this?

I don't know. Never been a brother, inexperienced. However, I feel that this matter depends on age. If my younger brother and sister were very young, my brother would hold my sister with his little hand out of the mentality of protecting her. These two look lovely. If I grow up, my brother should not have such close physical contact with my sister, but more just psychological protection and language care.