"Her shoes are white."
Translation:Hendes sko er hvide.
Because "sine" along with sin and sit can only be used on the object of a sentence. And can only be used if the object belongs to the subject if the subject is a third person, singular pronoun or noun. "Hun har sine hvide sko" (she has her own white shoes) is different to "Hun har hendes hvide sko" (she has her [someone else's] white shoes)
Well, in this sentence there is only a subject, and since "sin/sit/sine" only goes with an object it wouldn't be right to have it in this sentence. However, in a sentence like "She has her white shoes" both "Hun har sine hvide sko" and "Hun har hendes hvide sko" should be accepted, if not then report them