What are some checklists for younger sisters?

First of all, she isn’t my step sister. She is one of my close cousins and like my very own sister and as an older brother, ig there’s nothing bad if i help her with her periods and boyfriend. She is a small child who probably doesn’t have much knowledge and/or experience and probably need help, guidance, support, etc. Why should I just stay out of my sister’s room? And by encrypted messages, I meant like whatsapp. What’s wrong with a brother and sister whispering? Isn’t it cute? Now, tell me how I sound like a weird creep or even a creep.