My mausi(technically) is younger than me and my sister?!

So my mum and sister are currently in my hometown for 2 weeks. My Grandfather's, youngest brother's, daughter( i.e my moms cousin, we also call them mausi too) is younger than me and my sister too(im 17, my sis is 13). Huh???????? What the actual f*** is this shit????? Imagine i call a 10 yr old as mausi. Tf.