At the bottom of 10a in Yevamos, Rav Ashi says to Rav Kahana that even without "De'e" it is possible to have Imo Anusas Aviv falling to Yibbum to her son. Rav Kahana answers that Achosah D'Heteira Komyree, D'Issurah Lo Komyree. Rav Ronesberg changes the Girsa from Achosa with a Hay at the end to an alef at the end. Is this a different word (such as Achvosoh)? and does this now mean that brothers (rather than sisters) are only discussed B'heteira and not B'issura? If that is the p'shat, I think I would understand it better. If not, what does this case have to do with whether the sisters are b'heteira or b'issura?
Thank you very much
As you say, there are no sisters in this case. Rather, the Yavam is a brother through Isur Kalaso.