Ok.. lets go back to the start.
i set up the airport express and connected via an ethernet cable in my apartment
when i connect it knocks the family off of the main router
What is your main router?? Do you have a separate router to the one in the house of the owner?
When you connect the express to it.. did you reboot the entire network.. this is always necessary to fix the IP addressing.
Are you doing double NAT in the router.. if so are you doing it correctly using different IP addresses??
I guess the other question is, why the express should be any improvement over the current poor connection by wireless to the owner's router.
The express is far from being a great wireless bridge and its signal is poor.
Have you located the Express in the window of the best possible room in the house with direct line of sight to the owner's house?
What is the signal level it is getting?? If you plug directly into the Express.. no second router does it work.
You still need to give me full details..and screenshots of the setup if you want me to help.