Probably not, if you use the Classic AIM or AIM Triton. I haven’t seen any Add-ons or plugins for these. If your sister has no reason to be on the computer, and if you use windows 2000/xp, you could lock up the computer while you’re away….

other IM progs out there such as Trillian and GAIM may have this feature. I’m not too familiar with them. GAIM is open source, so maybe you could suggest it and somebody will inplement it in!