This isn't complicated. Barack Sr.'s parents are in Kenya. Barack Sr. is in Hawaii with Stanley. Barack Sr. is corresponding via mail with his parents in Kenya, writing and receiving letters from them, and he's verbally reporting to Stanley what is in those letters.
but if both were in Hawaii, then why was Barack Sr. writing to Stanley????
He wasn't. Nothing in the text indicates he was. What makes you think he was?
She wrote: Your father wrote back.... Then you were born... Sounds to me that she is speaking to Barack II otherwise she would have said your grandfather wrote back.
That’s how I read it!