Mi mamá

Imagine a brother talking to his sister. He mentions 'his mother'. Would you assume that his sister is his half-sister, that she has a different mother? Now imagine a wife who is speaking to her husband and mentions 'your child'. Would you assume that she is not the child's mother?

¡Ojo! Here are some familial references I've heard in Mexico:

• Said by a brother to his full sister: Ya conoces a mi mamá. You know how mom is. (Instead of Y conoces a nuestra mamá)
• Said to his full brother over the phone: ¿Me pasas a mi mamá? Could you put mom on (the phone)? Note that the English translation doesn't require an article at all.
• ¿Ya hablaste con mi mamá? Have you talked to our mother yet? (The speaker asked his sister this on the phone.)
• Said by a wife to her husband, the father of their child: tu hijo (instead of nuestro hijo).
• Said by a son to his mother, who is married to his father: ¿Qué dijo mi papá? (instead of ¿Qué dijo papá?)

In English, when you say your mother it's understood that the person referred to is not also the mother of the speaker. As the examples above show, that's not necessarily so in Spanish.