one way is to uninstall their PPS and Funshion from their computers but I doubt they would let you...
My mom uses them too and I become very frustrated when she has them running as it takes so much bandwidth.
Upgrading to Broadband 50 wont help you much since the programs will still take as much bandwidth it needs to stream the drama that your dad and sister are watching LOL
You need to set up a QoS in your router, it provides different priority to different applications