"My little sister" says it all Nica.
You must have so many soft feelings for her.
It does hurt ....she is like your child...in a way.
I am so sorry.
What ever you do, don't be too hasty to set her
straight.
She will resent it. ("you are not the boss of me!")
haha darn kids anyway!
Pretend that she is older & wiser than you.
Get her to straighten you out..ask questions,
praise her. All I can think of at this moment.
Best of luck
clarity