Yeah room for a sister to Murdock, Alasdair and Merida. The deal with the sisters is that their spouses must not be told the family secret. The women are also encouraged to either become - or at least try to become an animagus but if she married within a year or two of finishing her education she might not have bothered.

Since her siblings know her well enough Murdock may have insisted she take an unbreakable vow just to keep her in line, like the women who marry in - which I guess could make her a little tetchy with them! lol!