هل يصح زواج رجل من امرأة إذا كانت أخته قد أرضعت أم المخطوبة؟

Scholars have differed on accepting the testimony of a single nursing woman to establish prohibition (of marriage). The majority do not accept it, while the Hanbalis accept it if the nursing woman is trustworthy, truthful, and just. This is the view given preference for fatwa. Therefore, if your fiancé’s sister testifies that she breastfed your mother five full and satisfying times, and she is trustworthy, then your fiancé is not permitted to marry you; because you would be considered in the حكم (ruling/status) of his sister’s daughter through suckling, and what is prohibited by lineage is also prohibited by suckling.