It is.
She was introduced in MK3.
Sindel's storyline begins tragically 10,000 years ago, when her kingdom was invaded by Shao Kahn, and she was forced to watch as her husband King Jerrod was killed, her people enslaved, herself becoming Kahn's wife through a forced wedding, and her daughter with Jerrod, Kitana, was adopted by Kahn as his own daughter shortly thereafter. This proved too much for her as a mother, wife, and queen, and soon after the invasion, she committed suicide to escape what she believed was eternal subservience to her new husband, but Kahn wouldn't let his new Queen go so easily and held her soul in Outworld by an unknown means, stopping her transition to the afterlife.
She is resurrected, brainwashed by Kahn into forgetting what really happened, and would rule along his side as her Queen of Outworld. Later she would learn what really happen, turn back to good etc.