It depends on your state. Some states allow it, but others do not, even if you gave permission and she was in her own home.
These are the states that allow underage drinking on private property with parental consent:
Alaska, Colorado, Connecticut, Delaware, Georgia, Illinois, Iowa, Kansas, Louisiana, Maine, Maryland, Massachusetts, Minnesota, Mississippi, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, Ohio, Oklahoma, Oregon, South Carolina, Texas, Virginia, Washington, Wisconsin, Wyoming
These are the states that allow drinking on private property WITHOUT parental consent:
Louisiana, Nebraska, Nevada, New Jersey, Oklahoma, South Carolina
If you're not in any of these states, then it doesn't matter whether or not you consented or her brother was there.