Why are Catholic priests called "father" and Catholic nuns only "sister"?

Maybe because “Mother” was taken by Mother Theresa.

They are consistent among monastic life. Monks are called brother, nuns called sister. Leaders of monasteries are called father, leaders of convents are called mothers.

Nuns aren’t the female-equivalent of priests, they are the female equivalent of monks.

It’s about status. Friars are referred to as “brother” and high ranking nuns become “mother” or “Mother Superior”.

I did not know that, thanks. I assumed they were treating nuns as second-class citizens.