Tierney pleaded guilty to the four counts in October. She will begin serving on Feb. 28, according to the Boston Globe.
The plea deal -- which wasn't binding for the judge -- called for a lighter sentence of two years of probation and a $2,500 fine.
"There must be an actual sanction," Judge William G. Young said today, over objections from prosecutors. He said a 30-day jail sentence is the norm for the charges.
After Tierney is released, she'll serve two years of supervised release, including five months of house arrest.
According to the Globe, Tierney's money management for her brother included paying bills for Eremian's three children and their sick mother.
John Tierney has said his wife was misled by her brother, who told her he was operating a legal software business, according to the Boston Herald. Tierney, who represents a district on the North Shore of Massachusetts, has served in the House since 1997.