How did my niece's face end up on my daughter's body after being backed up to Google photos? We took family pictures with my S3 which came out fine.  After checked the backed up photos in Google photo I notice that my niece's face had been transposed onto my daughter so that it looks line my niece is sitting next to an identical twin. It really freaked me out. The picture is fine in my phone's photo gallery.
This sounds like Google+'s Auto-Awesome smile feature. If you took multiple similar photos then Auto-Awesome will attempt to create a composite picture with the best smiles. Unfortunately in your case it appears to have thought that your niece and daughter were sufficiently similar that it put your daughters smile onto your niece.

You can see more information (and an example) on this site