Having happened upon Lunete imprisoned in the chapel at the spring of Broceliande Forest, and promised to defend her next day in trial by combat, Ywaine journeyed on in search of shelter for the night and found a strong new fortress with the land laid bare outside its walls. The people of this castle did their best to honor their noble guest, but kept breaking into tears until he insisted they tell him their trouble.

His host turned out to be Gawaine's brother-in-law, having married the great knight's full sister. They were being persecuted by a giant named Harpin of the Mountain, who had captured their six sons, killed two, and was threatening to murder the other four the very next day, unless the parents turned over their daughter for him to give to his vilest servants as a sex toy. Gawaine would surely have come to his relatives' aid, but was absent in Gore and knew nothing of their situation. Ywaine promised to fight Harpin if he came to be finished off in time for the knight to get on to his previous pressing appointment.

Fortunately, Harpin arrived with the captive sons just as Ywaine was distressfully feeling he could wait no longer. (All in all, it was a very busy day for Ywaine and his lion.) On leaving the rescued family safe, he requested Gawaine's niece and surviving nephews to take the dead Harpin's dwarf to Arthur's court and greet their uncle in the name of the "Knight with the Lion". This they did, arriving two days after Guenevere's return from Gore.

Counting Soredamors and Clarissant, this gives Gawaine at least three sisters in Chrétien's work. Being unable to find any name for the one in Yvain, and a little unwilling to list her simply as "Gawaine's Other Sister"; Phyllis Ann Karr coined the name "Alteria" from the Latin for "other".