Your sister always knew you loved when she wore socks around you ... she'd even caught you taking a sneaky sniff but never told you.
Until she discovered one morning that you'd shrunk after taking one too many whiffs at her dirty socks ... seeing you there right next to them outside of the dirty laundry basket.
She thought to herself:
"If he loves the smell of my socks this much, why not keep him, no longer as my brother, but as my foot slave ... "
"I'm sure he won't mind, and would actually love the idea of spending more time as my personal footrest inside the dirty socks he loves so much".
Then proceeded, without even uttering a word, to pick you up and toss you inside her right sock, then put her foot inside ...
"Don't go dying on me in there, you're mine now"
"I knew you loved my feet and smelling my socks, caught you a couple times before ... enjoy your new life, I'll keep you as my foot bug"