Many people have secrets that they only feel comfortable telling their sister. It may well be that their best friend is their sister and they feel closer to them than any of their other friends. It could also be that some secrets are best kept in the family and other people may not necessarily be able to understand them if you explained it to them. That’s why sister quotes say that sisters are so valuable. You might have an embarrassing habit or have a guilty pleasure that only your sister knows about.
The problem when it’s your sister is that sometimes they can use that secret to make sure you do or don’t do something regarding your parents. They could threaten to tell your secret if you get them into trouble with the parents. This applies in adulthood as well as childhood. The secret is safe with the sister as long as you don’t do anything to inconvenience her life!