Season 8, Episode 6, “Baby, It’s You,” a 2-part crossover episode with “Homicide: Life on the Street.” Might have been a stepmother. The episode summaries don’t say.
But I just thought of another one, Skin Deep, in that one a professional photographer of models is stabbed. One of his models is a suspect, and her daughter is played by Claire Danes. But I think in this one, the daughter was jealous of the mother's looks.