Well let's be honest... I don't recall by now... did Frisha officially (as in front of Mira) proclaim her relationship with Dave to be over?
I mean Emily knows as they talk about it dureng the 3-somes.
There is a chance Mira thinks her sister to be a slut and what ever Partner she has is someone else.
On the other Hand Mira and frishas beds are on oppsing sides of 2 rooms.
Frisha keeps exploring the house if MC is not visiting her but Mira stays in her room... she could indeed be oblivious of Frishas affairs
Well, we know Frisha told Emily during one of bed time cuddlings, because MC was present there.
We don't know if Frisha told Mira about ending relationship with dave, because there was no such talk between girls with MC present.
We can only assume Frisha told Mira about it "off-screen" or Mira, also "off-screen" asked Frisha about the little commotion near her room, to which Frisha answered she ended the "relationship" with dave. Or Mira just assumed thier "relationship" is over, because Frisha is always present in the house and dave doesn't bother to visit anymore, yet didn't confirm it.
Mira doesn't think about her sister about being a slut, but knows Frisha is a slut. I think it was during the "family bath" Frisha openly admited to being a slut. Also girls are taunting eachother about taking thier turn with MC and him being a still a "fair game" during bath.
So my guess is Mira is suspecting MC of doing whole house or even know it already, just have to catch them in the act and ask if she can join for a threesome or foursome.
I mean, who would believe Frisha is so clumsy she is dropping her spoon(?) for the 69th dinner time in a row?
Sure, thier beds are quite far apart. About... 10 meters apart and in different rooms, but those walls are not made with soundproof materials, but stone, clay and wood at best. Only deaf would not hear how Frisha and Emily are moaning like cheap whores when MC is doing them. Even girls themselfs are joking about Mira not showing up to all that moaning.
As for Frisha "exploring" and Mira being always in her room, that's just both engine and Chyos limitations, but also Quality of Life at the same time. There is no point for NPC's to explore and walk around. It would make only harder to find them and follow thier daily routine.