You know empirical problem solving can work backwards, right? Because we see the direct result we can then form multiple hypotheses as to what happened, then eliminate the incorrect ones through logical deduction of events.
If you see a camp fire do you need to know explicitly how it was started to know the fire's going to be hot?
Just because we don't know exactly what the deal with the parents is does not mean we can't define Tomoe and Mikage's relationship.
A glaring example Mikage's not really in love with Tomoe is that she constantly tries to acknowledge the fact, like she's trying to convince herself of it. Plus there's none of the cliches like blushing, ect ect ect.
