I think it's perfectly fine to use their indirect interactions and even the lack of animosity to extrapolate on what their relationship is actually like, and for the most part it makes sense.
Even if Kaguya doesn't hate Mokou as much as she does, that doesn't mean they aren't able to directly interact in any fun or interesting way, and honestly, that way of just
telling what their relationship is like instead of just
showing it, to the point of them not even ever talking to each other, is a bad choice by ZUN, and it really just makes both characters feel more one-dimensional and boring as a result (Especially for Kaguya, considering she even never talks about Mokou at all!), regardless of what their relationship is like.
I'm normally fine with ZUN making things vague, but doing something like Kaguya and Mokou's relationship this way, and
never having them ever directly talk in canon, frankly just feels lazy. I don't think it should have been the the fanbase's job to flesh out something that could've very easily been shown in canon.
No offensive meant with this post, that's just how i feel about it.