Do I need to wait the approval letter after I got the acceptance letter from the immigration department?

The acceptance letter from USCIS means only a confirmation that your sister's petition was received. It does not give you any rights or privileges. Most importantly, it does not give you any status with which you can stay in the U.S. or permission to work here. You have to understand that, if you are in the U.S., you cannot stay here waiting for your immigrant visa unless you have some status - a student visa or a work visa. If you entered the U.S. on a visa, an immigration officer at the airport stamped your passport and marked in it the date until which you were permitted to stay in the U.S. You can apply for an extension of your stay or for a change of status, but you cannot remain in the U.S. after the end of your permitted stay just because your sister filed an I-130 petition for you. If you do, and overstay 6 months, you will be barred from the U.S. for 3 years; if you overstay 1 year or longer, you will be barred for 10 years. It means that, when your sister's petition will be approved, and your turn to receive an immigrant visa will come, you will not be given a green card until you leave the U.S. and live in your country for 3 or 10 years. As to whether an acceptance letter is enough, there is a long road between it and the green card. First, USCIS will take approximately 5 years to process your sister's petition. Then, if the petition gets approved, it will be transferred to the National Visa Center of the Department of State. NVC will keep the petition until your priority date becomes current. Many people find this concept difficult to understand. Let me try to explain it. Every year, NVC can issue only a limited number of immigrant visas in each category. For brothers and sisters of U.S. citizens, this number is 65,000 per year. These visas are spread evenly to all countries of the world. There are many times more petitions filed every year; so there is a wait time, and the visas are given to the immigrants in the order of the dates when their petitions were filed - "priority dates". Also, there are many times more U.C. citizens who have brothers and sisters in Mexico, China, India, and Philippines than in any other country in the world; so these 4 countries have their own separate waiting lines. You will get your immigrant visa only after all the people who filed their petition before you. Right now, immigrant visas are being issued to the brothers and sisters of U.S. citizens who filed I-130 petitions in September 2003 ( in January 2003 - if the beneficiary is from China; in January 2001 - for Indians; in April 97 - for Mexicans; and in February 1993 - for Filipinos) If your sister filed the petition for you in October 2015, and if you are not from one of the listed countries, your priority date will become current in the fall of 2029, and National Visa Center will begin issuing visas to the people who had their petitions filed in October 2015. It will then send your sister a letter requesting certain documents and payment of visa processing fees. After NVC will receive from your sister all the requested documents and payments, it will send your case to the U.S. embassy in your country. The embassy will send you a letter asking you to come for an interview. After the interview, if the consular officer decides that you are eligible for it, the embassy will give you an immigrant visa. After you come to the U.S. with an immigrant visa, you will send certain papers to USCIS, and it will send you a green card.