Morgause

Mordred
When Morgause is sent to Arthur's court by King Lot, her husband, to spy, she is seduced by Arthur. Arthur unkowingly sleeps with his sister which leads to their son, Mordred.

Lamorak
She has an affair with Lamorak, the son of Pellinore. Pellinore killed her husband, Lot. This makes her son, Gaheris, angry and he decapitates her.

Lot vs. Pellinore
Since she is affiliated with the Orkney boys as her mother, it is seen as a betrayal by her sons that she has an affair with Lamorak who is connected to Pellinore.