How do I petition for my fiancee's daughter after we are married?

I can prepare the forms for both your fiancee and her daughter.

Unless you know what you are doing, it is always recommended that you hire an attorney. Meanwhile, you start by filing an I-130 petition for your minor daughter after you are married. It takes about a year or so for an appointment at the DR office.

You may petition her as long as she is under 18 at the time you marry her father.

If you are a US citizen and you marry your fiancee, you can file an I-130 visa petition for him/her. Whether they can apply for an immigrant visa or adjustment to lawful residency depends on their background.

You can file an I-130 petition for your step-daughter. If the I-130 petition is approved, then your step-daughter will process through the US Embassy in her home country to get an immigrant visa. Upon her arrival in the US, she will be a Legal Permanent Resident, albeit conditional in a nature.

The first question I have is whether you are a U.S. citizen. Lets assume, for purposes of your question, that you are a U.S. citizen. The second question is whether your fianc is presently in the U.S. Lets assume your fianc is in the United States as you imply she is present by your comment regarding counting both your income and that of your fianc. If your fianc is currently in the U.S. then getting married and adjusting status based on her marriage to a U.S. citizen (you) is likely the best option. Lets also assume the daughter in the Dominican Republic is unmarried and under age 21. The next step would be to file the appropriate forms on behalf of your fiancs child in the Dominican Republic. Eventually they would be issued Visas in their native country(ies) and enter the U.S. Financially speaking, you (the petitioner) would need to have an income and/or property that satisfies the current poverty guidelines based on the number of individuals you would support. With respect to the amount of time required I cannot give an estimate at this time.

You must file a separate I-130 visa petition for your step daughter. She will require an affidavit of support by you for her adjustment/visa application. You can include your wife's income if you resided together for 6 months and she worked lawfully in the U.S.

You will have to petition for the future stepdaughter. Alternatively, the stepdaughter could come in at the same time as the K1 fiancee visa, under a K2 visa.