It depends on the background, if it's the same in both pics (as in identical) then yes, this should be doable.
Also - I'm not sure what features Elements has as I use Photoshop.
You need to Quick Mask the head that you want to cut out, then move it to the other photo and position it. The basic steps:
1. Click the Quick Mask icon.
2. Paint the parts you want to select. Zoom in to 200% and use a fine brush around the edges. It will paint it Red - that's ok.
3. Once it's all painted, click the Standard Mode (or unclick the Quick Mask button, whichever you have) and then Select Inverse.
4. Hit Control + C, open up the photo you want to move your selection to, and hit Control V. Now position the head where you want it using the Move tool.
5. Use the Blur tool (not the Blur Filter, the Blur tool) to smooth the edges a bit to make it look like it's part of the photo.
If you have no idea what I'm talking about try one of two things:
1. Google for a "quick mask" tutorial until you find one that works for you.
2. Tell your sister to get lost.