I struggled with this in my childhood years because my younger sister is a redhead. She had curly ringlet red hair and if i tell you that NO-ONE ever looked at, or complimented me, it would be an understatement!
My mom used to say that people would die for my hair (it was really long down to my bum, lovely condition etc) but when i had everyone else in the world swooning over my sister's curly locks, it did really really affect me. I was never jealous of my sister's hair, i would have hated it, it was a big frizzy afro of hair! But it was different, and people couldn't help but notice it.
I wish i had better advice on how to combat such a thing, because children do pick up on it. I suffered greatly in my teenage years with self esteem issues, but i just found my own confidence as i grew up. I know i am no less attractive than my sister, in all fairness, it was always just her hair that got her the attention, and still, she has grown up hating it, and now wears it cut above her ears and always straight. My Nan even told her once that her hair 'used to be so nice' and that she had 'spoiled it' - so it's kinda come full circle. I am now easily the more confident of us both.
You can't control what people say to your children in terms of compliments, and i know for a fact people are stupid and do not think of how swooning over one child can really hurt the other. Just because my sister was a cherub faced, ginger haired 6 year old did not mean that i did not exist. I think my mom complimenting my hair really helped me, i know i didn't believe it so at the time, but looking back at my old pictures, my hair was incredible and i can only hope that it grows so long and lush again!