Not safe. You can get eye infections from sharing contacts. If you're gonna do it though, make sure you clean them really well. Not a good idea though.
Urgently need coloured contacts? You do know what urgent and need means right? Is it life threatening if you don't? Will it prohibit you from making it to prom? Personally, i feel coloured contacts just take away from your natural beauty. You can get an eye infection, and I don't think that's worth it for one day with fake lenses.
If there are not proscription contact lenses they are made for your sisters eyes only ¬ meant for your eyes. if the lenses you can buy off the shelf then you should be ok.
well you can, but clean them properly with the solution but don't make it a habit.