Even though they applied at the same time, the "backlog" at uscis caused her 7 year old daughter's application to take 10 months longer than my wife's. As I understand it, the 90-day window is based on when the green card was received, not when applied for, meaning we have to submit 2 different 751s, one for my wife now, and one for her daughter in 10 months. Is this correct, or can we include her daughter on the mother's 751?
If your daughter was approved more than 90 days after your wife's green card was approved you do need to file two separate I-751's. That is unfortunate as it is going to cost you a second filing fee but that is the way that it is. You still should list all of your wife's children on the I-751 but you do need to file a separate Petition.
You will need two I-751s.
You should retain an experienced immigration lawyer to review all the facts, advise you, and handle the case. You can find one through http://www.ailalawyer.com.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
Under these facts it will appear that two petitions will be required and each must be documented to stand on its own. Consult with an experienced immigration attorney for advice on how best to proceed.
While this answer is provided by a Florida Bar Certified Expert in Immigration and Nationality Law, it is for general information purposes only and an attorney/client relationship is neither intended nor created. You should seek out qualified counsel to review your case and provide you with advice specific to your situation. Call +1-561-478-5353 to schedule a consultation with Mr. Devore.