You can use the Content ADvisor.
In IE, click Tools - Internet Options
Click the Content Tab.
Then click to enable the Content Advisor.
Now, it gets a bit tricky.
Click the ALLOWED SITES Tab. (yeah, the Allowed Sites)
Type in the Site you want to block and
click NEVER.
Click OK and password protect your Settings.
The second way is to put it on the REstricted List.
Tools - Interent Options - Security Tab - Restricted Sites
Type in the website, and click ADD.
Now, this will block the site, but if she's capable, and knowledgeable, she can remove it from here. Using Content Advisor is the better way to go.
Same for the Hosts file. If she knows how to edit it, she will. Let's face it, if YOU can edit the hosts file and she's using the same Profile, then SHE can edit the hosts file.
You should be good to go just by using Content Advisor so long as she is not capable of downloading another browser to use on your machine.
I highly recommend that you set her up with her own profile on that machine. Not an Admin Account, but a power user, or limited. This can be controlled by you, and the IE settings can be set as above. In addiiton, Not being an Admin, she will not be able to download programs, or install programs thus she will not be able to load a new browser (or any other program for that matter.)