Why is Lunete so eager to keep Yvain and Laudine together, since their relationship seems to be dictated by convention anyway?

After Lunete makes Yvain invisible and thus saves his life, he falls in love with the fair Lady Laudine. She cannot see him and does not even know he is in the castle. Lunete admires the young knight's bravery. As she ministers to Yvain's wounds, she decides to help him further and thereby help herself.
When the castle's master dies from the injuries Yvain inflicts, his wife enters mourning. She might continue as mistress of the domain, but that would be unlikely if there is a legally designated male heir who should succeed. Having a husband would help her stay in control. If Laudine were demoted, Lunete would likewise suffer in status. By talking up Yvain, she makes her mistress predisposed to choose him, and thus improve her own situation as both of them would be beholden to her—as indeed occurs.