Maybe but I think having no strong role models of the similar gender does that.

It depends I guess. I idolized my brother who was 10 years older than me, and I wanted to be just like him. I dressed in nothing but boy clothes and sometimes even thought I was a boy until I was about 13...then puberty hit :) Maybe it depends on how close you are to them.

yes I believe so.

I don't know that it has that much of an effect.  I grew up with both brothers and sisters (two of each) and I'm kind of half and half... can be girly, but mostly a tomboy

I had two older sisters and two younger brothers, I was a tomboy. But I have a friend who is definatly more girly then me and she grew up with two brothers and lots of male cousins. I am definatly more of a tomboy then girly, even today.
My brothers spent a lot of time with me, it didn't make them girly. My sister's husband grew up the youngest of 7 and the only boy, he is definatly not girly.
In the end you have to wonder. Does it really matter?

i don't think so. I had 2 sisters, and a younger brother, and i was a tomboy.  My brother had 3 sisters, and isn't feminine at all.

My mom grew up with 2 older brothers and a whole bunch of boy cousins around her age, the girl cousins were either much older or much younger than her and she was a total tomboy and till this day she relates better to men than women. I've also seen the opposite, my cousin has 2 boys and a girl, one boy is older the other younger and she is a total girly girl, everything in pink and has to match the hair tie and the shoes and her gameboy and what not. 

Growing up, I had 6 brothers and 5 sisters (4 of whom lived with me) and I'm a tomboy in some ways but not completely. I mean, if a girl does grow up around boys, it seems that the majority may rule out and her mom may not show her ways to be a girl. Children are complex and I believe that it's up to the parent to take invested interest when such things as this are the case. The parent (whether it's the father or the mother) has to take the excluded child and find ways to make him feel as though he belongs whether it's bringing more men & boys or women & girls around the child so they can have someone like them around whether it's cousins, friends, school mates, etc.
I mean, when a girl is born in a family of boys she is more likely to become a tomboy because she wants to have a sense of belonging by them because she doesn't see much of what she should where she spends most of her time. So, the same is for a boy or a girl and regardless of what their predicament may be. 

Horse has 16 nephews and 6 nieces. None of the nices in my husband's family are tomboyish at all. They know they are women/girls and don't try to act like the opposite sex.  Maybe that has to do with their moms being around them all the time, or maybe it has to do with being raised Muslims in a Muslim country where the schools are segregated by sex and the sexes don't interact with each other except through strict blood relations I don't know.  I have two little sisters and a baby brother and my brother does not act girly.  He acts like a boy through and through. 

I had three brothers and kind of wanted to channel the tomboy vibe. That being said, I didn't have a hard time transitioning into girly activities... It does also make me think less stupidly. (i.e. not buy into all that ridiculous Cosmo nonsense)

I grew up with two brother, both older then me. and i ended up a true tomboi and still to this day its just a natural thing for me and i do still have tomboi tendancys. Like some said above i could relate to males more then i can with females. Dont get me wrong when i go to the clubs with the ladies i tend to go all out in my heels, nice sexii outfit with my nails done and hair done up. but im more comfy in sweat pants and tshirt =) lol

My brothers are 5, 8, and 10 years younger than me, but I showed signs of being tomboyish long before they arrived. I liked playing outside, climbing trees, that kind of thing. Then again I also loved My Little Pony and Barbies. Boys can be very rough and tumble, my brothers were, and I think girls do become a part of that. Survival instinct perhaps. It comes in handy now because I know how to handle a guy. Punched a guy once who tried to mug me. I attribute that to having brothers who liked to mock fight and some real fights mixed in. Now that I am grown, I enjoy fashion and shoes, but I also love video games and sci-fi movies. I like the mixture.