your both not going there
you can get married now, and it will not change your orders at all
they are not gonna give her orders to japan, just because you got married or cancel your orders
joint spouse has rules, like how long you have been on station, how long you have been married.
and just being married , doesn't mean you are eligible for BAH-- married service members are treated as single service members unless they are both stationed at the same post
2. so best bet is go ahead and get married and process the paperwork with your S1, and both enroll into joint spouse
then when she is eligible for orders-- they may send her to japan or they may not-- again, there are rules to joint spouse- so depending on your rank, her rank , your mos, her mos-- joint spouse may not even be possible