"Your aunt and my mom are sisters."
Translation:Twoja ciocia i moja mama są siostrami.
Whether it's formal or not, it shoud be accepted anyway, as it's one of many versions of the word that means "mother". You can't just exclude some words and justify it with the fact that "it's too formal". Cause if it is, in your opinion, for every other translation, then what's the point of existing of this word in Polish? I suggest that it should be taken as a right answer.