On your Niko/Sho connection, I read somewhere that Sho was experimented on by Kirijo Group. So it would be possible that Niko and Sho are related in some ways, even if not in blood. Minazuki does seem to talk about a girl he wants to protect, so maybe Minazuki is some sort of trauma left on Sho because of Niko being hospitalized and dying which got manifested as a different personality on Sho.
It's been mentioned in the Arcade Modes that either of the two Shos has somehow been wronged by the Kirijo Group, yeah, so a connection is possible there. If they are connected, it would be weird for him to not be that missing older brother, since Niko went out of her way to state that she was not the first child. It's a suspicious thing to integrate in the script, really.
However, as for the "girl" thing with Minazuki, this has been called into question now. You see, the term Minazuki uses in these win quotes in the Arcade Mode is "Ano Ko". "Ano Ko" is usually used to refer to girls, yes, but it can also be used to refer to little boys under some conditions... and some official screenshots from the translated version of P4U2 make it look like Minazuki was, in fact, referring to Sho himself. Yep, looks like he considers him a cute, little kid. Who'd have thought?
