Question about capitals

I'm writing a story using the words, grandma, grandpa, grandparents, etc. Other than the first word in the sentence, when should they be capitalized?
A few examples please.

He even went fishing with my Granddad and Ben.

I was an only child and raised by my grandparents.

I still stopped by to see my grandparents regularly.

My grandparents kept their memories alive

to meet up with Rob, his Grandson.

for being part of my Grandfathers life

Needless to say Grandpa always writes about you.

I said to Gramps

Here are a few actual sentence or part of the sentence.
Thanks
DG Hear

DG Hear said:

I said to Gramps

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This one gets capitalized because it is being used as a name. The others don't get capitalized.

sunandshadow said:

This one gets capitalized because it is being used as a name. The others don't get capitalized.

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I would also capitalise "Needless to say Grandpa always writes about you" for the same reason.

As I've always understood it, you capitalize monikers like mom,grandpa etc. when they can be replaced by actual names. 'Mom and Dad said' but not 'my mom and dad said'.

Mom and Dad can be replaced by Adam and Eve, but you can't say, my adam and eve'. My is a good sign to use lower case. You can't write ' my Mom and Dad' like you can't write, 'my Adam and Eve.'

I think sunandshadow said it right, when the title can be replaced by his/her name, capitalize. If not, don't. As soon as you write 'my', think lowercase.

'My grandparents'-lower case - 'Grandpa and Grandma said' should be capitalized.

(edited to add: 'his grandson' is the same type as 'my grandpa' - no caps.)

Bramblethorn said:

I would also capitalise "Needless to say Grandpa always writes about you" for the same reason.

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Yep, this one too. A direct name substitute.

Thanks for all the help. Much appreciated.
DG