Iva really doesn't care about Miat, but she recognizes that he's her ticket in, so to speak. Her only goals at this point are to get some treasure and prevent the orc from looting human settlements. She's trying to manipulate him into allowing her to come along, as she doesn't have any overt skills to provide him as Arbok and I'lara do.
The way I see it, Miat is the character most likely to "lead" the party based off of his knowledge and personality. He's very self-aggrandizing and apparently has the most knowledge of the keep. I'lara would also be a suitable leader, except that she isn't as bold as Miat is. She's much more genial. Through the character's observations thus far, it's reasonable to believe that Iva doesn't really see I'lara as anything more than an existence, while she sees Miat as a magnetic force as everything has gravitated around him thus far. At least that's the viewpoint that I've been using for her.
She's trying her best to look and act attractive so that Miat not only wants her along, but is likely to defend her if conflicts pop up with other party members (specifically, Arbok). She doesn't trust Arbok at all (she's not racist, just to clarify that -- she's just incredibly intimidated by him) and she thinks that I'lara is doing the same thing that she is in regards to Miat. When she heard people discussing cuts of treasure she wanted in on that, thus this. That's why I'm asking if I should be rolling a bluff check, as not only is she trying to manipulate Miat, but she's also playing stupid and downplaying herself and her abilities.
Realistically it shouldn't really work as Iva doesn't have a great Charisma score, so that's why I'm wondering if we should do a Bluff vs Sense Motive check.