for my sister's immigration application (F4) and they ask me to fill DS-261 for online choice of address and agent.
Do I must choice a agent or not? If not, how complicate will be in future to process the visa application?
If I must use a agent, how much the cost usually is?
-You misunderstood what the agent means. The agent means someone your sister chooses to represent her to receive the NVC's letter for her in US. You can be the agent (not need to hire a lawyer). So Just put your name as the agent and your address in the form and ask your sister to sign the form. Then, send it to NVC (you can email the completed form to your sister, then she can print it out and sign it, then she can scan it and email the scanned form to you)
Thanks