<speculation>
I'd expect that a situation such as this would have been covered in one or the other of his biographies, perhaps both. I have to wonder if it was ever mentioned in either of them.
I'm uncertain as to when he wrote his books in relation to the timing of his mother's death. As far as I can tell, both books were written in the 2000's, well after his mother had died. Does anyone happen to know the initial publishing dates of his biographies and whether the circumstances regarding his mother's passing was covered in either of them?
</speculation>
Per the link to wikipedia I posted earlier:
He originally planned to finish the book in one year, but it took much longer as the book evolved into a personal memoir. In order to work without interruptions, Obama and his wife, Michelle, traveled to Bali where he wrote for several months. The manuscript was finally published in mid-1995 as Dreams from My Father.[22]
I can’t find it right now but I seem to remember reading that his mother read his 1st book before she passed away (Nov 1995) and made a comment to the effect of she wasn’t bothered that she wasn’t in it much.
It looks like the Audacity of Hope was published in 2006 (interesting he had so much time to write the book w being a Senator, running for Pres and all).