महात्मनः great self, अस्य of this, सुग्रीवस्य Sugriva's, धरमाणस्य when he is alive, पापकर्मकृत् a sinner, कामात् due to lust, स्नुषायाम् in your daughterinlaw, रुमायाम् with Ruma, वर्तसे live.

'You are a sinner, since you live with Ruma, who is your virtual daughterinlaw, when the great self Sugriva is alive. (A younger brother is like a son and his wife, a daughterinlaw).