Canto 2 -
Ayodhyā-kāṇḍa
Chapter 8: The Conversation Between Kaikeyī and Mantharā
Text 2.8.37

दर्पान्निराकृता पूर्वं त्वया सौभाग्यवत्तया।
राममाता सपत्नी ते कथं वैरं न यातयेत्॥

darpān nirākṛtā pūrvaṁ tvayā saubhāgyavattayā
rāma-mātā sapatnī te kathaṁ vairaṁ na yātayet

darpāt = proudly; nirākṛtā = had disregarded; pūrvam = in the past; tvayā = you; saubhāgyavattayā = because of your good fortune; rāma-mātā = Kausalyā, the mother of Rāma; sapatnī = co-wife; te = your; katham = why; vairam = retaliate; na = not; yātayet = would she.

In the past, because of your good fortune you had proudly disregarded your co-wife Kausalyā, the mother of Rāma. Why would she not retaliate?

Mantharā thought that Kausalyā would retaliate by making Kaikeyī her maidservant.