My advice is based on your initial reason(s) for divorcing your wife in the first place, and includes the responses you give to the following two questions...

1. Did your wife divorce you during your first marriage to be with your brother because she cared for him?, or...

2. Was their marriage one based on her needs to obtain a green card &/or citizenship based on your brother's U.S. citizenship status?

If the honest answer to  question "1" is "yes", then case closed --- no need to go any further.  The reason being that if she is just flip flopping back and forth between the two of you, she can not be trusted and is definitely not marrying material.  However, if the answer to question 1 is "no", and to the second (i.e., question "2") is "yes", then I say to pursue your heart's desire, and try to build a healthy relationship here in the United States.  Just be forewarned that the immigration department looks rather closely at the validity and history of all immigrant couples applying for citizenship, and may not be too impressed with your wife's past marriage history --- especially if you two were married and divorced previously in the United States.  They too will think your wife to be fickle when it comes to marriage, and will weaken both your chances in someday becoming U.S. citizens.  Good Luck in your pursuit of happiness and legality of citizenship!!!!!!!!! 

p.s. - Lily gives accurate information regarding the rules and regulations required for both green card distribution, and subsequent citizenship requirements.  However, her very final comment may not still be necessarily true.  In the past this would have been perfectly true, but I recently read about a deportation case involving a mother who had given birth to a baby here in the United States, but not only did this not help her with her immigration issues; it also did not automatically make the baby a U.S. citizen as this type of U.S. birth would have automatically provided in the past.  Laws are presently toughening and are now being interpreted differently every day so be aware of this trend.