Can you go from Tiger to Snow Leopard or do you need Leopard first?
Can you go from Tiger to Lion without intervening updates?
I'm getting the specs on her laptop.
Hmmm... I believe you can go directly from Tiger to Snow Leopard, but you'd better wait for a "definite" reply.
As for Lion... If your sister has an Intel Mac and is running Tiger, there's a chance that her Mac is not up to
Lion specs, particularly:
General requirements
Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
If she is up to specs, she must install Snow Leopard before she can upgrade to Lion via the Mac App Store; she
may be able to avoid Snow Leopard with the
OS X Lion USB Thumb Drive, but I don't think trying to go from Tiger to Lion would be a very good idea.