I highly doubt that we're going to suddenly find out that the MC is related to any other characters in the game, personally. Now, I COULD see finding out that his mother came from one of the families that founded the school that Isabella went to. I don't see him getting an inheritance, though, even if offered. The guy loves the heck out of his dad, but his mom's parents essentially disowned her for being with him. That alone would probably leave him conflicted if the grandparents suddenly wanted to meet him or give him anything, assuming that the letter has anything to do with them at all.
I do kinda expect the MC to fall into some money due to his mother somehow, though. Maybe she had some stuff in her father's and her name that got sold, and the MC was awarded some of the profit due to her being dead. If he does get a bit of money, I wouldn't be surprised if we have to make a difficult choice on what to do with it. The MC isn't the type of person to keep it himself, so maybe you have to make a choice on who gets it, like other people have suggested.
The choices would probably align with the possible relationships, like the tuition for Maya so that her and Josy (and you if you chose it), allowing them to escape Maya's father and just be happy without having to hide it. You could give it to Sage to get the HOT's out of the mess that Quinn has created, because I'm 100% certain that Sage actually knows what Quinn's up to; hence the party in episode 5 being for Quinn, NOT the HOT's. For Jill, maybe it would have something to do with the project that gets mentioned a few times that she's trying to set up. Perhaps something that she can't get herself for whatever reason. Isabella would probably have something to do about her missing husband. I'm sure fixing the DIK mansion and helping your dad would be in there, too.
Or, I'm just pulling things out of my behind (100% likely) and just like speculating a little too much.
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