If you're good with drawing, you can sketch your room. Then photocopy it many times, and start colouring until you get something that looks pleasing.
Sozobe's idea is very good. You could also do the room in blues, with green accents (bedspread, blankets, cushions, lamps, picture frames, etc.) and pink for your sister.
If you aren't good with drawing, you can take a digital photograph of your room, and print it in black & white. Then do the colour thing on that. But there may be too much grey, so you may need to trace over the B&W photo to get a line drawing that you can use.
Or you can go
here (select the 'paint now' button) and select a room that most looks like yours, and play with the real paint colours. The problem is that there is no way to save the photo when you do get something you like ~ although you can print it if you have a colour printer.
Have fun!