Are the contacts in phone memory or in sim memory?? If in phone memory, There's no problem. Switch on the phone and manully delete all contacts. There's no need of the sim to be inserted. If the contacts are in sim memory then insert the sim. Switch the phone on, and scroll contacts. Chek and find the no.s which have the sim image, it will surely be a sim contact. Then delete it. If you are fearing that your contacts ( if in sim memory ) will be deleted and it is important then first switch on the phone with sim. Note down all sim contacts, after that delete. Then if you give your sister another phone which she likes, switch on the phone and then install all sim contacts which you earlier noted down.