Looking to extend a wireless/wired signal from my parent's house next door.

Hey guys I just honestly have no idea what I'm doing with this and I don't even know where to start. Basically I'm looking to leech off of my parents' internet next door, but the wifi signal is too weak for their router to work in both houses. I currently have my own comcast service at my place and my parents also have comcast service. Recently, my dad ran a cable wire from his house over to mine. We plugged MY rented comcast modem/router into my dad's cable and I have full and fast internet. We theorized that if I simply got a non-rental modem/router then I would be able to have "free" (for me at least) internet. I went and bought a new modem router and I've come to find that I need to call comcast and have them activate my router. I have no clue if I'm going about this the right way or if I just need something different like a wireless repeater. The thing that is baffling me is my rented comcast router is working with my parents' line right now, and that opens up a whole bunch of questions: Is my current rented modem only working on my parents' line because I'm paying for internet on my old original line? Could I simply activate the new non-rental modem router on my line and account, then cancel my comcast service and plug my modem into my dad's line? Is another modem/router even what I need? Thanks guys. BTW I know that it might be technically against comcast's terms of service but Im not too worried about that. Thanks guys and girls.