Not mysterious? The woman lives 86 years and dies like two days before her grandson is elected president? Color me skeptical.
Thomas Jefferson and John Adams both died on the exact 50th anniversary of the Declaration of Independence, aged 83 and 90. Some people are apparently able to "hold on" to life a little longer, even when they are very ill. Madelyn Dunham reportedly had cancer for a long time. She probably wanted to make it to the election but didn't. Lots of people who are in their 80's have cancer and die.
Conspiracy theorists want it to be true that Mrs. Dunham was killed to silence her, but if the Obama campaign wanted to kill her to keep her quiet, wouldn't they have wanted to do that a lot sooner, before polls indicated that Obama's election was assured? Also, why would she want to say anything to keep her grandson from being elected?