[POMPONIUS libro quinto ad Sabinum. ] §23.2.8.prLibertinus libertinam matrem aut sororem uxorem ducere non potest, quia hoc ius moribus, non legibus introductum est.
[POMPONIUS in the fifth book on Sabinus.] A freedman cannot marry a freedwoman mother or sister, because this rule was introduced by custom, not by statutes.