Edited: So Lea is the MC's cousin, she mention wanting to go to Germany, mention family there, in MNM their was a mention of Lina going to visit some family there and I think I remember a mention of gingers there too, so I suspect it was Lea, her mom and their extended family. I have to admit I thought Lea might have been the MC's half-sister, but I guess this idea sorta passed through my head too.
While interesting I feel this story in a way retcon MNM to some degree, their was a mention of Anna introducing the notion of the MC & his twin doing something intimate together in MNM, yet in this game you have Lea that was pushing for that pretty strongly. The notion was that incest in a way was Anna's kink, yet it feel like this game introduce the notion it was already to some degree something the MC's got into it before Anna was even back. MNM gave the impression the MC was alot less sexually active with others than this story imply, I mean he literally got 5 new LIs from this game, before he even got with Anna, it give off the vibe he was solidly a polyamorist before Anna was back, even if MNM seem to imply it came from her.
Next, the weird dreams, in MNM while it was mentioned the MC and Anna had a incredibly strong bond, at no point anyone mentioned to the MC that he had in the past similar weird dreams shared with Anna, yet it literally happen in this game, and he mentioned it to virtually everyone around him, so he should have had something similar already. It feels like this game will just about cause a requirement for the rewrite of MNM to account for all the changes this story narrative bring to that reality that seem to diverge from what is supposed to be its sequel. It really feel like all those relationships from this game should have been very important to the MC, yet they don't exist as far as MNM's narrative goes. I mean sure their are implications to some degrees for example I stated above that Lina likely visited Lea & her Mom in Germany, possibly also for Lena & her daughter to have been either visiting / staying over with Rei.
Like I had mentioned in another comment it really feel like by the end of this we'll hear that in MNM the MC had half a dozen more children in existence. It feel weird for all of that to exist yet for MNM to still remain as it is, so yea I feel a rewrite will be needed to adjust the divergeances between the two games, because right now all you can ask is 'where the Hell were all of them were in MNM and how come no one mentioned any of it'? I mean you'd expect just about everyone of them would rally over when they heard the MC got into a severe accident, they all love him, sure you can say 'oh they'll be there for Christmas' but it sounds odds none the less, also wouldn't their had been plans for a FAR bigger house considering the circumstances? I mean it gives the impression that at one point all of them should have wanted to return together again and considering the current expected family size, its clearly missing a staggering amount of space in their current house in MNM, its barely enough space for a third of the family together, less be said about all the other girlfriends, the MC's mother and quite possibly aunt, as well as his quite possibly dozen+ progeny.