Hello XXXXXXXXXX,
The rule for claiming the credit disallows a purchase from a close relative who is a lineal descendant. This includes your parents, grandparents, children or grandchildren.
You could not purchase a home that is also in your mother's name and still qualify for this credit. If the home were strictly in your step father's name, then you could purchase the home from him and still qualify for the credit. Here is what the IRS says about a step relative:
Step-relatives are neither ancestors nor lineal descendents and are therefore not related persons for purposes of the first-time homebuyer credit.
http://74.125.47.132/search?q=cache:YId9d0iJ-bAJ:www.irs.gov/newsroom/article/0,,id%3D206291,00.html+first+time+home+buyer+credit+relative+step+parent&cd=1&hl=en&ct=clnk&gl=us
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Thank you XXXXXXXXXX.
Accountant
25+ years tax consulting. Specializing in returns for US citizens living abroad