Kamasutra 4.2.18

4.2.18

ātmavṛttān tāṃs tad adhiṣṭhitān kuryāt//

The younger wife should regard the elder wife of her husband as her mother, and should not give anything away, even to her own relations, without her knowledge. She should tell her everything about herself, and not approach her husband without her permission. Whatever is told to her by the elder wife she should not reveal to others, and she should take care of the children of the senior even more than of her own. When alone with her husband she should serve him well, but should not tell him of the pain she suffers from the existence of a rival wife. She may also obtain secretly from her husband some marks of his particular regard for her, and may tell him that she lives only for him, and for the regard that he has for her. She should never reveal her love for her husband, nor her husband’s love for her to any person either in pride or in anger, for a wife that reveals the secrets of her husband is despised by him. As for seeking to obtain the regard of her husband, Gonardiya says, that it should always be done in private, for fear of the elder wife. If the elder wife be disliked by her husband, or be childless, she should sympathize with her, and should ask her husband to do the same, but should surpsass her in leading the life of a chaste woman.