"Your aunt and my mom are sisters."
Translation:Twoja ciocia i moja mama są siostrami.
You use "to be" with instrumental and "siostrami" is instrumental form of "siostry" (sisters).
Major oversight... added, of course.
I'm surprised that there were only three reports o.O
Because "matka" is too formal for "mom". Frankly, it's even too formal for "mother", but well, there's no better equivalent.