Complex sentence: Adele arrives, having helped herself to one of Rosalinde's dresses; her sister Ida, a dancer, is very surprised to see her there and swears she didn't send Adele the invitation.
Split into simpler sentences: Adele arrives, having helped herself to one of Rosalinde's dresses. Her sister Ida, a dancer, is shocked to see her there and swears she didn't send Adele the invitation.