Complex sentence: It was not unusal for a composer to set aside a project from losing interest, but in this case it was probably because of his sister Rosalie, who was the main supporter and the spokesman of the family.
Split into simpler sentences: It was not unusal for a composer to set aside a project, for losing interest. But in this case it was probably because of his sister Rosalie, who was the main supporter and the spokesman of the family.