I'm the 2nd of 4 kids, but I'm the only girl. I used hand me downs from my older brother clothing wise -- I was a MAJOR tomboy back then. One thing though, I've always had my own toys, my own clothes (when not in my tomboy phase), my own bedroom, etc. No sharing necessary.
I have to admit though, when I first got into make-up I bought those 88 palettes from ebay to see what colours etc I liked (turns out ALL the colours!) and I remember getting ready to go out one night with friends and they'd brought over this girl I'd never met... she asked if she could use one of the colours and I begrudingly said yes. B!tch dug that brush in there and nearly broke the damn thing, Heaux better believe I haven't shared since.
These days, I'll not share anything that's MAC, which is 99% of my collection as I admit to being a bit of a MAC whore and by the time I've spent the money on MAC, I've nothing left to spend on other brands. I just hate the idea of someone using something that's LE and breaking it and it not being able to be replaced as I just can't afford BU's.
Sharing isn't caring. Sharing it and having it not be broken when returned, THAT'S caring.