Its never been stated that she is.
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Asuma did not get married. He was in a relationship with Kurenai, and she was pregnant with his child. However, Asuma was killed before the baby was born.
Asuma and Kurenai were not related. They were lovers. Asuma was the father of Kurenai's baby.
In episode 89 of Naruto Shippuden, Shikamaru says to Kurenai, "When your kid's born, it'll be my turn to be the teacher." The camera then shows Kurenai's belly and left hand which has a ring on it. (she's either engaged or married)
It has not been revealed yet in the show or manga if Kurenai had a boy or a girl.
Kurenai loves Asuma. But there's a chance that she'll fall in love with another jounin since Asuma's dead.