Today when I was playing Mario Kart Wii in an online battle, I noticed quite a few of my female competitors were playing as either Peach, Rosalina or Baby Peach. To add to the point, my 13 year old sister always plays as peach in Mario Party games, Pipsy in Diddy Kong Racing and Rosalina in Mario Kart Wii. 

The same reason most men play male characters.

it's actually rather queer if you think about it.

I however always choose female characters in MK (preferably baby Daisy

Well I play as Peach quite a bit on Super Mario Bros 2, as her floating ability is very useful. I have also given her a go in the various Mario Sports games. Oh and I recently played as Cammy is Super Street Fighter 2 (she is not as good as Chun Li though), and I am a Male.

Well lets see. Maybe because they're girls.

Perhaps, maybe, it's because you aren't playing gamers, and so they just feel the need to select someone who they feel represents them.

However, when you play against gamers, they don't give a shit about their avatar, they just select the characters for the stats.

Where are you getting this figure? From what I've seen through years of MMO about half of the girls I've met play males the other half play females and where applicable some even play non-gendered but technically male characters(Galka in FFXI)

I guess it's possible in games such as mario kart and super smash bros females maybe have more varied ages than on games such as FFXI where it seems to sell to the teen-young adult crowd almost exclusively. And that younger girls may still be attracted visually to pink or other females, not in a sexual way! just in a visual way.

cause we like to use characters that represent ourselves more

I think it has more to do with your sister being 13 than anything, My friend always play as male character (or Kirby, which is a guy) in SSBB.

I play with Chin-Li sometimes in street fighter games