Is this really so taboo for a girl?! i'm effing frustrated right now!!!?

Depends, what do you want from them? If you want to just sleep around, screw around, whatever, then yeah, you gotta "flaunt ." But if you want to actually have a chance at ending up in a serious relationship with someone, then just keep doing what you're doing...if they think of you as a "little sister" then that means they're not thinking of you as purely a toy...i.e they actually care about you. And trust me, guys might flirt with the " toy" girl, but it's the girl they have a serious friendship with that they're gonna want to settle down with someday.