Speculating and Kissing
Nothing could prepare Lust to discover that Lucien's soul gates are linked to the Sins. That sounds so crazy that she can't stop talking about it and speculate how that is possible.
He, on the other hand, has his focus back on his girls because thinking too much about that isn't really going to give him answers.
Now, all he wants is to strengthen their girls and also help them look for soul gates within their souls. The fact that Sophia, Amelia, and Jeanne have already received a lot of his demonic energy and awakened their soul weapons makes them the most likely girls to find a second soul gate, but since they're not around right now, Oya and Helena could possibly do it first.