Complex sentence: Later that evening, Miss Marple talks with the sisters about what she thinks may have happened and, while they are doing so, Miss Barrow and Miss Cooke appear, to talk to Miss Marple.
Split into simpler sentences: Later that evening, Miss Marple talks with the sisters about what she thinks may have happened. While they are doing so, Miss Barrow and Miss Cooke appear to talk to Miss Marple.