Simplify the following text for a Grade 10 native English-speaking person.
Original: Although the treatment was initially successful , Kate relapsed ; ever since , Anna , the only compatible family member , has been used as a donor for any other bodily substance needed to treat Kate , who continues to swing between remission and relapse as she grows up .
Simplified: Although the treatment was initially successful , Kate relapsed ( became sick again ) . Since then , Anna had been used as a donor for any other bodily substance needed to treat Kate , who continues to swing between remission and relapse as she grows up .