Chick here. It's one of three options.
Either she's so comfortable with you that she doesn't even consider that you might think of her in a sexual way. There's a decent chance it's this, but most of us know that a guy is going to be turned on by a half naked woman no matter who it is.
Or she likes the idea, specifically just the idea, as a fantasy, of you desiring her, and she's acting on that fantasy. She wants to show off. She kind of half way hopes you're jerking off to her, but it's nothing more than that. As long as nothing happens, she can deny that it's real, and it's just a part of her mind's internal playground, getting the excitement of you wondering if you're getting off to her, without ever feeling guilty from acting on it. This is the most likely option.
Or, she's open to far more and is presenting herself to you to see how you react, trying to feel out if she's getting a positive response or encouragement from you to babystep it further. She's wanting to act out on the fantasy, but approaching it cautiously by seeing if you're open to it without the risk of jeopardizing her relationship with you. This is definitely possible, but is probably the least likely. A few mild comments to feel out her response could help you figure this out, like telling her that her panties are cute, or something as simple as saying that you appreciate she feels comfortable enough around you that she doesn't feel the need to cover up with a bra. Try to read between the lines in her responses.
For me, it's the second. It's a huge taboo fantasy, and I secretly hope my brothers jack it to me. It feeds my ego to think that maybe, just maybe, seeing me in a thong is to much for them to handle and they have to get off to the thought of me even though I'm their sister. If they tried to do more, I'm not sure how I'd respond. Part of me wants to say absolutely yes, and the other part is afraid I'd destroy my future relationships with my siblings forever. But in the meantime, I have fun being secretly sluty and hoping their eyes linger on me while they assume I'm just comfortable around them.