Manusmriti
ज्येष्ठो यवीयसो भार्यां यवीयान् वाऽग्रजस्त्रियम् । पतितौ भवतो गत्वा नियुक्तावप्यनापदि ॥ ५८ ॥
An elder (brother) who approaches the wife of the younger, and a younger (brother who approaches) the wife of the elder, except in times of misfortune, both become outcasts, even though (they were duly) authorised.
english translation
jyeSTho yavIyaso bhAryAM yavIyAn vA'grajastriyam । patitau bhavato gatvA niyuktAvapyanApadi ॥ 58 ॥
hk transliteration by Sanscript