EW3

Many people immigrate to the States by having the principal alien come first for a variety of reasons, and there is no disadvantage to having your father follow within the six months that he is given on the immigrant visa. There should be no issue with your mother and your sister coming in first. Neither would there be an issue with your father coming in later. The only rule of Immigration in this regard is that the dependent cannot precede the principal alien to the United States.Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.  

The visa that will be issued to the dependents should allow them to travel within reasonable period after the visa is issued.  They won't have to travel together if they are issued separate visas.  That can be arranged with the consulate at the time of issueance.