"She and my mother are sisters."
Translation:Ona i moja matka są siostrami.
It's another way to say "i" = "and". Polish Wiktionary says that it's good to use in writing, but it's better to avoid it in spoken language. It does seem a bit too bookish in many contexts.
As you can see in the default translation, you can say 'Ona i moja matka są siostrami'.
I see the default translation now, in the web browser, but I didn't notice it in the app. I must have declined siostry wrong