This explaination covers more than just the wireless piece, but:
As log as you have the wireless encryption setup on the router/pc's, you're ok/safe with the wireless piece. (And she is connecting to sites using SSL, 128 bit encryption when doing financial transactions)
You also must have the pc's in the house fully patched with Microsoft updates, running updated Anti-Virus definitions, and make sure (as best as you can) that they are spyware free (using Ad-Aware, Webroot, Microsoft, others). The only scenarios that you're still vulnerable to losing financial info (even if you've got wireless encryption setup and your machines fully patched/cleaned) would be by being comprimised by new/unknown Spyware (and Phishing) attacks.
Make sure (if you have kids for example) that they are not surfing questionable sites like free gaming/entertainment sites for example (or free porno sites
), as there are very often hidden "active content attacks" that can deliver key loggers, spyware etc...even by just
visiting the site. You don't necessarily even need to click on anything...there are such things as "drive-by downloads". I've known people that have done all the security controls and have still been comprimised because of seemingly bad "surfing habits" that exposed them to a new/unknown spyware attack.
Lastly, if you click on questionable content in emails, they can download and execute spyware, keyloggers etc. Or if you get tricked via email or a link in a webpage into going to a fake "Paypal", "Ebay" or "Banking" site and enter your personal information...that's called a phishing attack.
No matter how many controls you put in place and how patched/secure your systems are, there are still traps. It takes due-dilligence with keeping your systems secure, and exhibiting the right behavior with email and surfing on the internet to stay uncomprimised.
Even then you're really not 100% risk free. There is no such thing as 100% security. Do all of the above, and the odds are well in your favor.