आत्मवान् selfcontrolled, सः that Lakshmana, भ्रातुः brother, शासनम् command, श्रुत्वा on hearing, सर्वम् all, अप्रतिकूलयन् without acting contrarily, पूर्वम् first, मैथिलीम् Sita, आरोप्य having helped her board, ततः thereafter, आरुरोह got in

Having heard his brother's command the selfcontrolled Lakshmana did as he was told.He first helped Sita get into the boat and thereafter boarded it himself.