They obviously have something. Even if it's built into their modem, there must be an ethernet-out jack on it, if not several. It's probably something like one of these...
https://www.trustedreviews.com/reviews/talktalk-hg633-super-router
...or one of these...
https://www.trustedreviews.com/reviews/talktalk-wi-fi-hub
In which case, it could just be a matter of running a $20 length of ethernet cable from one of its ports directly to their computer upstairs. I have no idea what their house layout might be like, or where the modem might be positioned in relation to their room, so it's difficult to say what that might involve. In a simple scenario, they could just drill up through the floor in a closet and have that single cable come out the other end into their room. Or, for a more proper but complex installation, they might want to run cable up through walls with ethernet jacks at both ends. Of course, this wouldn't do anything for their phone's wireless connection, but would likely make network performance on their PC a lot more reliable.
As for potentially improving the existing wireless performance, is the wifi router located in a relatively central location in the house? If it's at the opposite end of the house relative to your room, or otherwise poorly positioned, that could negatively impact performance further away from it. It might be worth trying to move the router to see if that improves anything, if it happens to be practical to hook it up elsewhere.