how can my mother get a foreclosure off her credit report,she co-signed for my sister who lost her house and on her credit

This may not be the perfect solution but one thing she might do is to write a letter to each of the three major credit reporting companies with a short, concise explanation to be included along with the negative information. Simply explaining that the foreclosure was on a co-signed foreclosure will allow people who look at the report that although there may be derogatory information, there is a benign explanation.