The links by Cap should give you all the info you need, but the down and dirty is this:
1) Have your sister download and install the Access Runtime. She can download it from here:
http://www.microsoft.com/downloads/details.aspx?familyid=d9ae78d9-9dc6-4b38-9fa6-2c745a175aed2) Send your sister a copy of the database.
The Runtime can run a "standard" database, but of course it will not allow design changes and such, and there are quite a few caveats to doing this (see the link by cap above), the most important of which (IMO) is that you need to include full error handling (since the "default" error handling simply uses the standard VBA error handler, which isn't availabe in a runtime environment). This addin will help with that:
www.mztools.com.