I'm more inclined into komurosaki being kinemon daughter. Was Odin a samurai?
I am still hesitant to the claim of it being confirmed, she could just be the daughter of some government official under the Kozouki, who got slain during the rebellion after all. Moreover a "distant" relation like aunt 2nd grade (dont know the term in english) is still within the scope of possibility as well imo. No need for her to actually be the sister, but as u said there r several things hinting at that to be the case.
I think what kind of sealed her being Momo's sister is how in this chapter Orochi was talking about Momo still being alive as they never found his body and it cut to a panel of Komurosaki just being silent. Then earlier in a previous chapter when Komurosaki was introduced it immediately cut to Momo talking about finding his sister. It it feels like Oda is setting her up to be the sister based off those. It would feel like a bit much to have it all be a misdirection, even if I would prefer it to be one.