Complex sentence: Duffy was a supporter of the Treaty, which her brother had signed, and as such she left "Cumann na mBan" and joined "Cumann na Saoirse" which she was instrumental in founding as an Irish republican women's organisation which supported the Pro-Treaty side.
Split into simpler sentences: Duffy was a supporter of the Treaty, which her brother had signed, and as such she left "Cumann na mBan" and joined "Cumann na Saoirse" which she was instrumental in founding. It was the Irish republican women's organisation which supported the Pro-Treaty side.