Is Mrs Phillips (Mrs Bennet's sister) a widow? Because there doesn't seem to be any mention of her husband.

No, she is not a widow. Mr. Phillips is Mr. Bennet's clerk. You can go here:

www.scribd.com/doc/13970576/Pride-and-Prejudice-Study-Guide

Mrs. Phillips was one of the Miss Gardiners (one of Mrs. Bennet's two sisters)then she married Mr. Phillips. There is no noting of her husband being dead in the book, so I am assuming he is never really mentioned, or only mentioned once or twice.

no,actually mr.phillips is the one who introduces lydia and katy to the army men