Siwan, also known by her English name ‘Joan’, was a negotiator between her husband, Llywelyn the Great, and her father, King John of England.
Roedd Siwan, oedd hefyd yn cael ei hadnabod gan ei henw Saesneg ‘Joan’, yn cynnal trafodaethau rhwng ei gŵr, Llywelyn Fawr, a’i thad, y Brenin John o Loegr.