"Your mother's sister is your aunt."
Translation:तुम्हारी माँ की बहन तुम्हारी मौसी हैं।
मामी is also maternal aunt but she is your mother's sister-in-law (her brother's wife). So, तुम्हारी माँ की भाभी तुम्हारी मामी हैं। where भाभी is one's brother's wife.
[I know I already replied to you on another thread but just wanted to leave this here as well]