Haemophilia is a sex-linked recessive disease (X-linked) .

1. Analysis of the Mother: The twins are fraternal (derived from two different eggs). One offspring (the brother) is normal, meaning he received a normal X chromosome () from the mother. The other baby is haemophilic, meaning they received an affected X chromosome () from the mother (if male) or from both parents (if female).
2. Conclusion: Since the mother possessed both the normal allele (passed to the normal son) and the affected allele (passed to the affected child), she must be heterozygous (a carrier, genotype ).
3. Other Options: While the affected baby is most likely male (as female haemophiliacs are extremely rare and require an affected father) , the most genetically certain statement based on the segregation of alleles to the twins is the mother's heterozygosity.