relative pronouns: whose

Whose is a possessive relative word. It does two things:
a. it joins clauses together
b. it is a ‘determiner’ like his, her, its or their. Compare:

    I saw a girl. Her hair came down to her waist.
    I saw a girl whose hair came down to her waist.
    This is Felicity. You met her sister last week.
    This is Felicity, whose sister you met last week.
    Our friends the Robbins — we spent the summer at their farmhouse — are moving to Scotland.
    Our friends the Robbins, at whose farmhouse we spent the summer, are moving to Scotland.