A plot twist might be that Lily actually has a change of heart towards Snape after many years of misunderstanding. She realizes that the boy she knew in school was much more complex than she thought. Maybe she sees his loyalty to her in a new light, like when he constantly protected her from the Marauders' pranks, not just as a one - sided infatuation on his part. And so, she decides to marry him. Also, there could be a magical bond that forms between them accidentally. For instance, during a dangerous spell - casting experiment gone wrong, they end up magically bound to each other, and through this experience, they develop deeper feelings and end up getting married.
One possible plot twist could be that Lily marries Snape out of a sense of duty. Maybe she discovers some long - hidden secret about Snape's past that makes her feel obligated to stand by him. For example, if she finds out that he made a great sacrifice for her family in the past that no one else knew about. Another plot twist could be that it's a marriage of convenience. The wizarding world is in danger, and their union is seen as a way to bring two powerful magical lineages together to fight against a common enemy. Or perhaps Snape has some unique magical ability that Lily needs in order to achieve a certain goal, and marriage is the only way to access it.