The Dasaratha Jātaka says Gotama (our current Buddha) was Rama in a past life and his wife (Gotama's son's Rahula's mother Yaśodharā) was his sister Sita in the same past life. In this past life, Rama appeared to marry his sister Sita (who, according to the translation, became Rama's queen-consort). Then later Rama was reborn as Gotama and Sita was reborn as Yaśodharā (Rahula's mother).
Was Gotama's marriage to Yaśodharā an example of 'incest'?