It could be that this is a phrase from a work of fiction that is not following normal language or ethical norms. Without more context, it's hard to determine exactly what it is supposed to convey. But it gives the impression of something that is not in line with good taste or proper language use.
If we break it down, 'daughter' is a family relation term. 'Swap' usually means to exchange. But when combined with'sex story', it doesn't seem to be a common or appropriate phrase. It might be a case of someone creating a very odd or inappropriate - sounding term that doesn't have a legitimate or positive meaning in normal language.
I'm not entirely sure what this specific phrase means. It could potentially be a very strange or made - up term. Maybe it's a very specific reference within a certain fictional or niche context that is not widely known.