Why is Rahel so chilled by Chacko looking at the photo of his daughter?

Comrade Pillai shows Rahel a picture of herself, Estha, Lenin, and Sophie Mol. Rahel remembers that Sophie had made herself look silly for the picture, adorning herself whimsically and making a funny face. Just before the picture was taken, she had explained to Estha and Rahel that there was a good chance they were all illegitimate children. The picture was taken days before she died.