Years ago when I was a teenager my mum sadly died. My dad remarried a couple of years later and my new stepmum tried to insist I didn't call her by her name, but called her Stepmum.
I found it weird, stepmum is an adjective never a proper noun right?! I did sometimes as she made a fuss if I didn't, but in the end just call her by her name and she has accepted this.
Anyway, I sometimes ponder whether anyone knows anyone who calls their stepmum 'Stepmum'.