Sometimes you don't need a "real" database to accomplish what you need.
Unless you'll be getting into queries and Customer Relationship Management or other data-heavy activities, maybe there is a very simple alternative your sister could use.
If she has Microsoft Outlook installed (on an up-to-date system with all of the security patches and so on), she may be able to do well with the Contacts "database" that comes with it.
Perhaps even a "spreadsheet" program would suffice, like the one included with OpenOffice.org or Microsoft's Excel. Simple columnar data.
She might do a search for "contact manager" or "pim" (personal information manager) on a shareware or other download site and explore some of the offerings she finds. Start with the freeware ones and see what you find.
I just did a quick search on a venerable download site and discovered a $240 piece of software specifically for managing a beauty salon. They say "the software is so user-friendly that your staff will require little or no training".
There are lots of alternatives to "real" databases out there. Don't overlook the simple things.