Raeanna stuck posters around her primary school in Gateshead calling on pupils to join her in a fundraising run to raise funds in support of children and young people with cancer, and their families.
Her letter said: 'We are all very worried about Caitlin, however we are not raising money for her we are raising money for everyone with leukaemia.'
Raeanna got hundreds of children running with her around a park near her home in Gateshead, raising £3,500 for CLIC Sargent.
Raeanna said: "When I first found out my sister had cancer was the worst part.
"Caitlin was coughing and getting wheezy and I kept getting scared so I locked myself in the bathroom. One of the reasons we raised money for CLIC Sargent is because they were there for us.
"It's great the amount that has been raised and hopefully it will help a lot of families."
Caitlin and Raeanna's mum, Lisa, said: "When you talk about leukaemia with most children they might not pay any attention but Raeanna is a bright girl and when she understood it was a form of cancer that's when she got upset."
CLIC Sargent Fundraising Manager, Lindsay Kay, said: “We would like to say a huge thank you to Raeanna and her family and the pupils and staff for this fantastic donation.
"We really appreciate the hard work and generosity that has gone in to raising the pounds – every penny raised will help provide vital support for children and young people with cancer, and their families.”