|So, having been summoned here to give a review i'll give it a go.
Firstly, I just want to say that I like yugioh and that Seto kaiba is quite a cool character who does care for mokuba a lot. And i think that the way you've portrayed them here in this setting is honestly better than I was expecting as a lot of fanfiction writers mess up the characterisation. I think you've done it well.
Secondly, the character of Joan seems to be a kind woman and is a very natural character, like a person who is living and has things to do. (definitely could have worded that better)
On that note, the dialogue too is also very natural and believeable so i commend you on this as it is something I struggle with personally.
I do have something to suggest though regarding dialogue which is that you have too much dialogue, i think. And to improve, you could provide more description of the surroundings to help the reader become immersed or maybe to make the characters more active.
Overall, a good read that fans of yugioh and maybe a bit of drama may like. Keep writing!