Is it bad that my teen still calls me mommy?

My sister kind of makes a big deal that my 15 year old still calls me mommy. She thinks it means he's still super dependent on me or something stupid. He's actually very independent. He actually spends a lot of time doing his own thing and hanging out with friends. Idk why he still calls me mommy instead of mom. He just never switched it up. She says I need to cut off th umbilical cord. I don't understand really what her issue is. Hes just a sweet kid. Hes not afraid to hug me or say I love you in front of his friends, but he also does what he's supposed to do, does well in school, has his own hobbies, so its not like he's attached to me. Idk what do you mama's think?