According to Wikipedia, Heleni HaMalkah was married to her brother. Since she was a convert to Judaism, on a Torah level, her familial relationships were severed. However, as far as I know, there is still a Rabbinic prohibition to engage in incestuous relationships after conversion; she seems to be violating a Rabbinic law.
But wait! The gemarah (Sukkah 2b) quotes a beraita in which Rabbi Yehudah says about her:
כל מעשיה לא עשתה אלא על פי חכמים
She always kept the Rabbinic laws
How is this contradiction resolved?
Possibility: the Wikipedia link cites Josephus as the source that Queen Heleni was married to her brother. Perhaps we simply do not accept this as accurate?