punkvegita, what I intended to say was that even though as small children they were not responsible for their actions, once they turn 18 and leave and re-enter and are questioned about their prior visits, they should not conceal any facts that they have knowledge of, because at that point they are adults trying to conceal factual information. So participating in fraud as children cannot be held against them, but covering it up as adults probably could.
For your brother, I think a US citizen spouse could file a petition right away, he just has to be outside the country for 3 years until he can return (it would make maybe 6 months difference if she applied right away or after the 3 years).
USCIS may be suspicous of a marriage of two 17 year olds but it all depends on the evidence they can present for their marriage of course. In that case it may be in your brother's best interest to get a lawyer's opinion whether his arrival constituted a legal entry allowing him to adjust status (then he may not have to leave and come back), but again for that he'd need the full information of what happened during initial entry.