Kidnap Naru and kill Nephrite
The Plant Sisters are three Youma who work for Zoisite who only appear in the first anime series. Their names are Housenka, Grape, and Suzuran. While none of the character's names are spoken in the episode, they can be confirmed in other materials.
In episode 24, when Zoisite discovered that Nephrite has fallen in love with Naru, and possibly betrayed the Dark Kingdom, he sent the Plant Sisters after Naru to kidnap her so Nephrite would come and save her, giving them the chance to kill him. Nephrite did come to save her (right after discovering that Usagi was Sailor Moon), and beat them up, nearly killing Grape (the one that shoots thorns from her hands) with a sword.
The Plant Sisters soon found Nephrite and Naru again, and Grape stabbed him in the shoulder with thorns, leaving some stuck inside him. Housenka shot fireballs at Nephrite and Naru, but Nephrite took the full force of the attack, and was already mortally wounded from the thorns. Before the Plant Sisters could kill Naru and Nephrite, however, Sailor Moon, Sailor Mercury, and Sailor Mars showed up. Sailor Mercury used her Sabão Spray to create a mist that stopped Housenka's fireballs and left them confused as to where the Sailor Scouts were, and Sailor Moon and Sailor Mars combined Moon Tiara Action and Mars Fire Soul, destroying the three Plant Sisters. Unfortunately, this wasn't enough to save Nephrite, as he later died from his injuries.
Grape is pink and has slight characteristics of a tree, and shoots deadly thorns from her hands. Housenka is yellow and green and shoots fireballs, and Suzuran is blue and shoots sonic waves.
This is the third time Sailor Moon and Sailor Mars combine their attacks to destroy a monster, and the second time only they do so (as Sailor Mercury combines her attack with theirs to destroy Castor and Pollux).
Even though fans remember the Plant Sisters as the ones who killed Nephrite, Grape is the true killer since she is the one who stabbed him with her thorns.
The lower half of the sisters' faces are never shown.