I do very much love those aus where Jonouchi and Kaiba become extremely disgruntled in-laws through a Mokuba x Shizuka marriage (siblingshipping... really? I feel like this name probably needs an update... maybe littleshipping? Because they're both the little siblings? Idk man) but they also accept that this is happening and they're technically family now, so they're kind of nice to each other in a passive-aggressive way about it
Seto: Jonouchi. It has been brought to my attention that you intend to show up to my brother's wedding in your usual work suit. I can't allow you to do that because you will stick out like a sore thumb, and you'd take all the attention away from the ceremony, so I'm taking you to buy a new suit.
Katsuya: Seto! So good to see you again! I heard you're taking Bakura as your plus one, so you won't mind if I take Yugi, right?
Seto: I suggested to Mokuba to avoid serving beer at the wedding. Of course, it's not because of your father, I just think sake would suit the reception better since Shizuka wants a traditional wedding. Yes, it's a nonsmoking venue, too.
Katsuya: So, for Mokuba's bachelor party, we're taking him to a dive bar since he's never been to one before. You're the best man, so you're coming too, right? You're not going to work though your only brother's big day, are you?
Then maybe next year they'll be the ones getting married, lol.