Cfxtrjtrk

Context
My brother and I when having a conversation that refers to our mother usually use "my mom" to refer to her. For example "Have you talked to my mom today?" is a common question we ask.

Now the discussion came up when some of our friends pointed out that it was weird and incorrect for us to say "my mom" since we share the same mom and are siblings and that we should just say "mom".
However, my brother and I think its grammatically correct to use "my mom" since its technically a factual statement.

I'm just trying to find out if our usage is incorrect in English grammar and if our friend is correct that we shouldn't use the pronoun "my" in conversations between siblings.

Mom and my mom are very different: it is not simply a question of possessives.

If you and your siblings shared, say, a truck, then you would simply use the appropriate pronoun:

[to stranger]: Where did I park my truck?

[to sibling]: Where did I park our truck?

Of course, you could apply the construction to mom:

[to stranger]: Have you talked to my mom today?

[to sibling]: Have you talked to our mom today?

This is still a bit unusual, primarily because Mom is often used as a proper name. For example, you might say:

Hi, Mom. It is so nice to talk to you!

However, the name Mom is usually used within one's immediate family. I might say "Have you talked to Mom today?" to my brother or even father. This is despite my father, clearly, having a different mother than myself. But I would not use the name Mom when talking to someone outside my immediate family; instead, I would say my mother.

Of course, this last point is not a matter of grammar. The question of who is allowed to use a term of endearment has no definitive answer; it depends on personal preference and cultural norms. Even within the English-speaking world, different people may have different standards for who uses the nickname Mom. To the best of my knowledge, my analysis is fairly standard for white Americans in the Mid-Atlantic. But other groups may have different usages.

It is not incorrect to refer to your mother as "my mom" when speaking to another of her children, but it is uncommon and it may convey the wrong information to speakers of most common dialects of English.

There are many factually accurate ways to refer to your mother in converstation with a sibling: "my mom," "our mom," "your mom," "our brother's mom" (if you have another sibling), "Dad's wife," "Grandma's daughter," "Our uncle's sister," "Gertrude," "the queen of Denmark," etc. Each of these names and descriptions is factually accurate, but conveys slightly different information.

For example:

Sister: "Did you ask Mom about our trip to the park?"

Brother: "Why don't you ask her? She's your mom too."

Sister: "Is she? She acts like I don't exist. But OK, I'll ask my mom."

Ordinarily, when something is possessed by multiple people, we use refer to it using our. If something is possessed jointly, it's true that it's also possessed by each individual, and so it can be referred to using my. Using my instead of our emphasizes your unique claim to the thing and in some contexts will be read as a challenge to the idea that the thing is jointly possessed:

"I don't like the color of our house."

"Our house? This is my house and I like the color of my house."

When you refer to a mother that you share with your sibling as "my mother," it sounds to other people like you're challenging your sibling's relationship with your mother ("she's not our mother, she's my mother").

It appears in your family's dialect, my does not carry the implication of and not yours. I wouldn't call this "incorrect," just a different usage. And it's different enough from most standard dialects that you're likely to confuse listeners.