My sister keeps getting my pictures saved into her photo

The following shows how to use iTunes to sync photos (and will therefor tell you where to look). However, the synced photos do not end up in the Camera Roll and I can't think of any way someone else's pictures would get into the Camera Roll. You may want to check again to see if in fact that is where they are. http://support.apple.com/kb/HT4236

Not possible under any circumstances for photos you capture with your iPhone or save on your iPhone which are saved/stored in the Camera Roll on your iPhone to be transferred to the Camera Roll on any other iOS device.

Not possible to transfer photos on a computer to the iPhone's Camera Roll via the iTunes sync process.

Possible to transfer photos to the iPhone from a computer via the iTunes sync process but not to the Camera Roll.

The only way to do that would be your sister having to constantly be restoring her iPhone with iTunes from the backup for your iPhone.

WELL... UNLESS ... her photos and your photos are saved somewhere on the computer you share, and you import all photos off your computer, and dont check the source locations you might find stragglers appearing

also depends too if the computer uses one itunes account and recognizes your phone, which it shouldn't, but if you sync via itunes with one account, that accepts both phones, its possible.

but in camera roll, i'm not sure if she's differentiating where her photos are located at all. if she can tell the difference between camera roll and albums in the iphoto, then you can rule out camera roll as the place where the stragglers are appearing.

Which photo album?

Do you share the same computer and iTunes that each iPhone is synced with?

If so, what do each of you have selected under the Photos tab for the iPhone sync preferences with iTunes?