Sorry, but they're a weird family. They're strangely controlling and vindictive. Maybe the SIL wants to live vicariously through you, since you have what she's never had (marriage and kids). Plus you're pretty, so I can see how she'd be extra jealous. Maybe you should start lowering your expectations of them. Seriously. That way, you won't get as mad when they do something stupid.
As for the hair thing though, if auntie couldn't respect my wishes when I'm being nice enough to let her see my kids to begin with, then I wouldn't let them visit her anymore.