We learn in this section, that he has at least one sister, possibly two, and a cousin who is close enough that Mrs. Jennings wonders if the letter that upsets him at breakfast is from her. Whitwell is owned by "a" brother-in-law of Colonel Brandon's--does that mean he could have more than one?

That brother-in-law is abroad, and Colonel Brandon's sister is in Avignon--the assumption would easily be that these are the owners of Whitwell, if it weren't for the indefinite article 'a' used with brother-in-law.