I thought the reason is that they don't like each other very much. I see more photos of Rei and Tomoe together than Tomoe and Mayumi. Maybe we'll get those two in a lesbian scene some day...
Something with Rei and Tomoe would also be great, yes! Or all three of them with Rei being the cushion between them in case they really don't like each other.
Imouto has done several IVs with two Idols, but I think this one would be the first IV with three of them.
Hell, why not all Imouto Idols in one big harem IV....production code IMHR-001
But as far as I remember I have seen a lot of pics with Tomoe and Mayumi together...and I don't mean official Oimoya photos or something like that, I mean private pics on Twitter. Still less than one would expect between two sisters, but surely more than of Rei and Tomoe together.
Maybe they don't love each other like twins would do, but I never got the idea that they really don't like each other. Maybe the constant competition of the past 10 years between them has done some damage to their sister relationship. It's difficult to tell from the outside.
Edit: The following three pics have just been posted by Mayumi on Twitter
in the past three days. They don't seem unhappy there together.
(Come on girls....finally do it
)
The left one comes close to what we all wanna see. The boobs are already approaching each other saying 'hello'.