Given the following context:

The story begins when Heidi, a little five-year-old girl, is brought up to the Swiss Alps by her Aunt Dete, who took care of the little girl ever since the passing of her two parents. Dete leaves Heidi with her grandfather, who lives alone up in the mountains, and though at first the old man refuses, Dete insists and he finally agrees to take her in but sends Dete away discourteously. Heidi's sweet, innocent nature slowly wins the heart of her grandfather and they soon grow fond of each other. Heidi wonders about every aspect of her new life in the mountains and has questions about all sorts of things. She befriends Peter, a young boy slightly older than she, whose job it is to lead the many goats up to the mountains every day. However, just as Heidi decides that she could never love anything more than life up in the mountains, Dete returns to take her away from her grandfather and to the city of Frankfurt in Germany, where she expects the girl to serve as a companion for the Sessemann's invalid daughter Clara. Though she misses her grandfather and the goats dearly, once in Frankfurt, Heidi becomes the best of friends with Clara and is saddened by her inability to walk. After many conflicts with Madam Rottenmeier, the strict care-taker at the Sesseman's, at her birthday party, Clara gives Heidi what she thinks she wants most: a train ticket to Switzerland, to her grandfather and old friends. Another surprise awaits Heidi and her grandfather as Clara and her grandmother come to visit them up at their home in the mountains.

answer the following question:

Who is taken away from her grandfather to Frankfurt? 

Heidi