"She and my mother are sisters."
Translation:Ea și mama mea sunt surori.
The sentence as a whole basically translates to "she, together with mom, are sisters" (or whatever would be the more grammatically correct version of English). I can't tell you why but I can tell you it is the more common/casual version of the two sentences. "Si cu" is a very common sentence building block in my experience.
I understand what you're saying, which I found among the possible answers. But on this discussion page, Duo translates the phrase as "Translation: Ea și mama mea sunt surori" (yet, as mentioned, not as a possible answer), which is what I would have imagined.
Why, oh why.