Who is correct?

[Elisheva]

DS's homework has been returned with one sentence marked as incorrect. He had to add capital letters and full stops to sentences and the teacher seems to believe that 'Mary went to her grandma's house for tea.' should have a capital G for grandma.
I disagree. What should I do?

Bump

You are correct.

I have just found this page which gives the following example:

Capitalise family words when they are titles or substitutes for a person's name:

Just then Mother called me on my iPhone.

The man at the edge of the photo is Uncle Pete.

.. but do not capitalise them if they are preceded by a possessive:

Have you met my mother?

The man at the edge of the photo is my uncle Pete.*

www.grammarly.com/handbook/mechanics/capitalization/4/capitalization-family-relationships-when-used-as-p/

You are correct, for any avoidance of doubt. As to what you should do, I'm not sure. No teacher likes to be corrected by parents. As a former teacher, I'd be more than happy to have this questioned by a parent but can tell you not all of my former colleagues would have been.

Recently, my 11 yr old niece wrote 'shouldda' on a piece of work and her teacher corrected it to...... "should of"

I would be totally unable to stop myself speaking or sending a friendly note to a teacher who wrote "should of". Just think, it's a chance to stop generations of her future pupils from growing up thinking that's acceptable.

You are correct.

Your post made me gasp, Eggbutty.

no not a capital G, a colleague of mine types called M for mother, which irritates me, as she is wrong

Grammarly upsets me. It is American. How so I know it's the same as English English?

Capitalisation for example in the link.

On this issue, the rules are the same for American English and British English. Grandma should be lower case in the OP's example.