just googled, not sure if it is true but seems she is from a middle class family.
http://www.dailymail.co.uk/femail/article-517824/Mrs-O-The-truth-Michelle-Obamas-working-class-credentials.html
Michelle's father was a volunteer organiser for the city's Democratic Party, a by-word for machine politics in America, and his loyalty was rewarded with a well-paid engineering job at Chicago's water plant. Even before overtime, he earned $42,686 - 25 per cent more than High School teachers at the time.