Your sister will not be able to purchase either OS X 10.5 (Leopard) or 10.6 (Snow Leopard) from Apple or other normal retail channels. Both are obsolete products. She might be able to find copies on eBay or at one of the on line stores specializing in old Macs, but there are precautions especially when buying from eBay. Be prepared to pay a premium price for these out of date operating systems.
  1. You must purchase a retail (black disk) version of OS X. The OEM (grey disk) versions that shipped in the box with a new Mac will only work on the specific Mac model they shipped with.
  2. Leopard (OS X 10.5) has both Intel and Power PC code
  3. Snow Leopard (OS X 10.6) and Lion (OS X 10.7) are only available for Macs with an Intel processor
  4. The Mac App Store is only compatible with OS X 10.6.6 or later.
  5. Lion (OS X 10.7) is very stable and very reliable. I have upgraded my wife, my sister, my daughter, my grandson, and my son-in-law to Lion and the number of phone calls for help have fallen to almost zero.
  6. Information on the Apple web site, not surprisingly is focused on currently shipping products, but their support knowledge base and forums go back several years. The trick is knowing how to search for the information you are looking for.

IMHO, if your sister has a Mac with an Intel processor she would be better off technically and financially upgrading to Lion.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein