you probably need to set up some iptables rules now. Your system probably dont know where to send traffic right now. So you have to nat connection from your parent subnet to the fast ISP and the one from you to the slow one. Something like :
iptables -t nat -A POSTROUTING -s 192.168.2.1 -o eth2 -j MASQUERADE
This will work as long as your parents gateway is bigpond gateway (you might want to ajust your DHCP server to send the correct address).
Or you could force their traffic to be routed to the ISP too, with something like
iptables -t nat -A PREROUTING -s 192.168.2.0/255.255.255.0 -j DNAT --to-destination 192.168.4.2
(but I really dont know how this will react with the masquerading, you will have to test).
With some time, you could really setup a nice router, with bandwidth throlling and dynamic routing