The sentence implies that she is already sitting. Like Jean Luke's example, adding a "down" would imply movement.

She sat down in her chair. The sentence makes it seem like she first sat in her chair, then drank coffee.

"Peacefully sipping her coffee" is a slow, relaxed activity. It is not an activity that you can do easily while you are in the process of sitting down. Therefore, she is already sitting in her chair. Since it doesn't make any reference to her picking up the cup or beginning to sip, she has been sipping coffee for a while and is going to continue to sip it for a while. Someone could say "She was standing in the office, sipping coffee peacefully. She put the cup down on her desk, sat down, picked it up, and continued to sip peacefully."

Su.Ki. is correct

Either option could be correct. The unseen sentence prior to the example given would help to give the context in which the sentence was used.