You may be referring to the tax credit of up to $7,500. That program is a program where you receive a $7,500 tax credit for purchases of primary residences from April 9, 2008 - June 30, 2009. That amount is paid back in $500 installments over 15 years.
I don't see anything pertaining to limitations due to buying a home from your parents. The only thing that might be a flag to the government would be if the purchase weren't for market value or otherwise looked like it was done to just get the credit. I know a lot about the program ... but haven't read the entire text though and there may be other fine print that I am unaware of.