My sister is four years older than me and I started smelling her underwear in my early teens. She caught me a few months in but didn't freak out about it and just said she should remember to knock. She's made a few subtle comments over the years intended to go over everyone else's heads but mine though. However she's never done anything sexual with me, except for some occasional very light teasing like asking which outfit looks the best via selfie or sitting with her legs uncrossed in front of me, etc. I would never try anything sexual either because I know absolutely nothing would ever come of it.
Luckily for me to this day when she goes on vacation or away for a work function she leaves me the separate key to her bedroom when I drop by to feed her cat for her. I like that it's our little thing no one else knows about.