Lulu knows Wakka like the back of her hand. There's a lot of friction between her and Wakka. But for some reason I couldn't help but think that perhaps she had seen Wakka before or during her relationship with Chappu. Lulu and Wakka were also guardians together. So maybe it's possible that since Lulu hadn't seen Chappu for a few months.... and she was with Wakka whome they were both guarding Father Zuke, they probably had time to clear their minds and play around with one another. I mean....after all. Why in the heck would you want to bed your dead lover's eldest brother and end up having his kids? Seems kind of weird to me.