I see Lillian's actions as as appropriate to what information we have about their society. Last chapter, Evris posits "Who's ever heard of (marriage) between two women!?" so there's not even a record given of, say, it being attempted but lead to being ostracized.
Her "betrayal" in taking Mark becomes something more of a sacrificial action, as Evris tells Ciel when leaving Mark's mansion that "He and I are the dregs of our houses, after all." There's no way that Mark could have ever made Evris happy, so while it was certainly a strange way of doing it, Lillian basically 'too the bullet' to save her.
(I'm also not exactly sure what her plan after that would have been; because unless The Prince came next then she would somehow have to continuously get in the way of Evris' engagements until he did. And after 2 that just starts to look spiteful...)
I imagine it being incredibly frustrating by holding back feelings for a dear friend from childhood because going beyond whether or not such a union would be taboo or not, the base concept for itself simply doesn't exist....and then this carefree ponce swoops in just two days ago and goes "LOL, love <3" and takes her away? She just got things worked out with Mark!