Gift money from wedding

Hello everyone, last weekend I got married, had a celebration with some guests and we ended getting around 1.8 million yen in gift money (my wife is Japanese, so on her side they were heavy on the money).

The issue now is that after counting it all, I don't know how to manage the taxes. Her sister said that she didn't declare anything extra when she received her money (although she does remember that at the bank they asked her where she got the money from, so maybe it was processed as a gift automatically and she paid the taxes without knowing? She has quite a good salary, so I could imagine her not noticing). So then, my wife says that we should just go to the bank and put the money in (we have a shared account, but it's under my name)

My main worry is that I started my kojin jigyo this year, and being my first year I can only go by my experience on how freelancers are treated in Spain, where they are treated almost as criminals and checked from top to bottom, so I am a bit scared. Also, the clock is counting towards PR, so I want (and need) to be extra clean with everything tax related.

I have thought about having her bring the money to her bank, that has a bank teller, and then transfer it to our shared account (we use SBI Net, so no offices available).

Any help is appreciated, thanks!

by old_school_gearhead