Responsa על נדה 95:35

Q. L, an orphan, was betrothed by her mother to A. Subsequently A was killed on a day of rioting and massacre leaving a year-old brother. L is eleven years old according to the testimony of women, and has no symptoms of pubescence. May L sever her dependence upon A's brother by "Refusal"?
A. We follow Alfasi (Yeb. 107b) who rules that a minor may sever her dependency upon her levir by "Refusal". In this case we accept the testimony of the aforesaid women regarding her age and condition.
This Responsum is addressed to Rabbi Yakar haLevi.
SOURCES: Cr. 81.

Q. A minor orphan girl was betrothed to A. She wants to exercise the right of "Refusal" (סירוב). May the testimony of female witnesses be accepted that the girl is only eleven years old and, therefore, may still exercise the right of "Refusal"?
A. The mere ceremony of betrothal, without cohabitation, is binding only because of Rabbinic enactment, but is not binding according to Biblical law. The testimony of women is acceptable in the annulment of a marriage that is valid only according to Rabbinic enactment. Therefore, since A did not cohabit with the orphan, the testimony of the female witnesses is to be admitted in evidence.
SOURCES: Cr. 286; Pr. 569; L. 389; Mord. Yeb. 60, 61; Tesh. Maim. to Nashim, 14.