Ben and Jo Sanders are taking part in the world-famous marathon in aid of The Stars Foundation for Cerebral Palsy.
The siblings are running in memory of their cousin Emma-Louise Easty, who had the condition.
Ben, from Cuddington, said: “Stars is one of only a few charities that helps people with cerebral palsy.
“Their work helps to make life easier for families.”
Ben completed the London Marathon for the charity in 2011 raising £1,000 but for Jo, who lives in Aylesbury, this is her first experience of running a marathon.
Ben said: “I decided I wanted to do another one and Jo wanted to do one so wedecided to run together.
“We are hoping to raise a combined £2,000.
“Training is going very well - we did a ten mile run last weekend and are looking to do a 12-13 mile run next.”
To donate visit www.justgiving.com/Jo-Sanders.