sibling petition

[antonio_needs_help]

Ok, Well here goes my question. I have a nephew that was brought here illegally by his GC holder mother that has now disappeared, so my 12 year old nephew is now living with my mother (his grandmother). My nephew has two older brothers born in the U.S. so they are citizens one of them just turned 21, so I was wondering if he can petition for my nephew being that he is here illegally and not actually living with his older brother? I also know that he would be 4th preference which means that his wait would be several years before he can get a visa number which might make him older than 18 by the time he gets a visa number would my nephew have to leave the U.S. so as not to be here more that 180 days after his 18 birthday?

Please help