Dick Francis's Bloodline

When race caller and television presenter Mark Shillingford calls a race in which his twin sister, Clare, an accomplished jockey, comes in second when she could have won, he believes the worst: that she deliberately lost, and the race was fixed.
 
When Mark confronts Clare with his suspicions, she storms off—and it’s the last time Mark sees her alive. Hours later, Clare jumps to her death from the balcony of a London hotel. Devastated and guilt-ridden over her death, Mark goes in search of answers. Why would Clare throw a race? What led her to take her own life?
 
Or was it not suicide at all?