All my BILs and SIL call my mom mom. I wanted to call my MIL mom, she was the most amazing woman. I loved her. She told some nurses in the hospital that I was the daughter she never had (she had 6 sons). But, it's not something you do in this country.
My mom calls my dad's mom 'mom' too.
If anyone has issues with it, roll your eyes at them and ignore them, or tell them to shut up, or just laugh at them and walk away.