Squall and Rinoa can't be half-siblings. They're the same age, so it doesn't add up.

Even if Laguna and Julia had a fling, there was a considerable delay before he even met Raine. He was badly wounded during his adventures in Centra, and spent six months bed-ridden while recovering. Unless Raine raped him in his sleep while he was still injured, and then hid her pregnancy from him... but that doesn't work either, since we know for a fact that Kiros came to find Laguna one year after the events in Centra. Later still, they left to find Ellone after her abduction.
So, Raine definitely became pregnant more than a year after Laguna's chat with Julia, since there was nothing going on between Laguna and Raine by the time Kiros got there.
In short, there's simply no way Laguna could be the father of both Squall and Rinoa.

Unless you buy into some stupid conspiracy theory like "Raine drugged Laguna and stole his sperm, then kept it in the freezer for over a year and used it to impregnate herself when General Caraway turned out to be impotent".

I'm sure Serapy or Future Esthar would have some... fascinating input in a thread like this