First of all, someone with a B-1/B-2 tourist visa can only request an extension one time to stay here for one year maximum, it is not something to renew every six months (one simply cannot be a tourist for years and years in the U.S.) Also, she must have good reasons why she wants to stay here longer as a tourist for another six months before the extension will be granted.
The problem with filing for her now when you're only a permanent resident is that there is no visa number (immigrant visa number, that is) available for a few years for her because it's a limited category. Only an I-130 can be filed for her and that is only a preliminary step that is not the same as a green card for her. If your citizenship case is denied (due to criminal records, lack of sufficient presence/residence in U.S., etc.) or drags on for more than the expected six months to a year to finish, she will not be able to adjust her status to green card later if she is out-of-status on the tourist visa. If you become a citizen, then there will be a visa number immediately for her and she will qualify for a green card then, but only a consultation with an attorney can tell you if your citizenship case will be smooth or if there will be potential problems. If it will be smooth, then you can start filing for her after you became a citizen even if she's out of status, assuming Immigration has not found out about her.
A qualified immigration attorney to handle your case is highly recommended to prevent delays which usually happen with self-help. Be sure to check out our blog with useful immigration info.
Larry L. Doan
http://guruimmigration.wordpress.com (blog)
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