So for the sake of argument let's say Japan does forbid marriage of in-laws (I don't know if it does or doesn't), and let's also say that Himeno & Ouji eventually get together and so does Ouji's brother and Himeno's sister. If one marries before the other, the other pair couldn't legally marry in that situation.
But what if both of them get married at the exact same time, by having a double ceremony where the same priest does the marriage for both at once? Neither would be married first, so neither was an in-law before getting married. That's an interesting legal question if nothing else.