If you always email your mom, dad and sister together, and you start composing a message to your mom and dad, Gmail will suggest adding your sister. You can click on through suggested names and add them as recipients. All this is possible using new Gmail labs feature ‘Suggest more recipients’.
To enable this feature, goto settings > Labs tab, then hit the enable button next to ‘Suggest more recipients’ and click save changes button. This feature is very useful for Gmail users who often email to group of people on regular basis (and tend to forget few recipients). Gmail’s ‘suggest more recipients’ should come handy in this regard.