Complex sentence: Lisa, Monika and Alexander--were removed from the cellar as infants to live with Fritzl and his wife, who became their foster parents with the knowledge of local social services authorities.
Split into simpler sentences: Lisa, Monika and Alexander--were removed from the cellar as infants to live with Fritzl and his wife. The Fritzls became the three children's foster parents with the knowledge of local social services authorities.