The recent Very (Un)Scientific Survey
produced pretty unremarkable results in terms of whether or not character preferences were influenced by having younger or older siblings - unsurprisingly (though I admit to being a little hopeful that there was some pattern).
What is a little bit interesting is the number of participants who are the eldest.Participants and their sibling relationships:
Of the 81 participants just over half had younger siblings. Unfortunately I can't make a sweeping statement regarding the demographic of Supernatural watchers from such a small sample of fans. I am sure this says more about my flist than anything else (though it would be totally cool to find out if this small sample was representative of SPN fans. I wonder what that would say about SPN watchers and being the eldest in the family).
As to whether your character preferences are determined by if you have older, younger or no siblings I think we can pretty confidently say there is no relationship whatsoever.
All the categories (Dean, Sam or Bi!bro) had majority of the eldest sibling.
Dean people: 55%
Sam people: 57%
Bi bro: 44%
(ETA: Poll results have changed - due to more participants. Though the trend still holds)
For full results check out the Poll.
In the comments most people agreed that even though they had leanings toward one brother they are essentially bi bro. A few ticked each box, arguing that their preference changed as the season progressed. Or felt it fluctuated.
I'm not sure if a full demographic survey of SPN fans has even been done. I know some media have referred to fans as "14 year old girls" - which feels completely off the mark to me. But that's probably just the circles I keep (and looking at con attendees). There may well be as many 14 year olds as 25, 34, 49 or 66 year olds - (with an even spread of older, younger or no siblings).
Determining what makes someone lean to either Sam or Dean might just need to be filed in the too hard basket...;)