Shloka 4.18.19
वाली–रामसंवादः (Rama’s Justification to Vali on Rājadharma)
अस्य त्वं धरमाणस्य सुग्रीवस्य महात्मनः।रुमायां वर्तसे कामात्स्नुषायां पापकर्मकृत्।।
asya tvaṁ dharamāṇasya sugrīvasya mahātmanaḥ | rumāyāṁ vartase kāmāt snuṣāyāṁ pāpa-karmakṛt ||
While the great-souled Sugriva yet lives, you—doer of sinful deeds—out of lust have consorted with Ruma, who stands to you as a daughter-in-law.
'You are a sinner, since you live with Ruma, who is your virtual daughter-in-law, when the great self Sugriva is alive. (A younger brother is like a son and his wife, adaughter-in-law).
Dharma treats the younger brother as son-like; therefore his wife is protected by a taboo akin to ‘daughter-in-law,’ making lust-driven appropriation a grave wrong.
Rama specifies the relational ethics: because Sugriva is alive and is Vali’s younger brother, Ruma is not a legitimate object of Vali’s desire.
Moral accountability for desire—kāma must be subordinated to dharma.