Manava Dharmashastra 9.121

9.121

upasarjanaṃ pradhānasya dharmato nopapadyate
pitā pradhānaṃ prajane tasmād dharmeṇa taṃ bhajet //

The representative (the son begotten on the wife) is not invested with the right of the principal (the eldest brother to an additional share); the principal (became) a father on the procreation (of a son by his younger brother); hence one should give a share to the (son begotten on the wife of the elder brother) according to the rule (stated above).