In this case it is because 'mother' is a close family relationship and singular. This site is really worth bookmarking http://italian.about.com/od/grammar/a/italian-possessive-adjectives.htm
I see this was posted a long time ago but want to comment anyway. NEVER feel silly for asking Andrew. I do not look at the discussions behind each answer (correct or incorrect) so nobody would know whether or not you read about this in another comment. Keep trying (as we all are). Remember, quitters never win and winners never quit!!