Oh derp, misread that bit, so it's a direct cable to your mum's PC.
Then you'll have no issues swapping the router out at all.
You might as well upgrade to FTTC, even BT's HH5 will be better than your current Netgear, although it doesn't have the bandwidth tracker you want. But the FTTC speeds and latency will be a massive improvement and another PC shouldn't slow down the rest of the network.
If your parents upload files, then FTTC will be a huge improvement as well. ADSL is limited to 1mb up and when that's saturated it can heavily affect your download/ping as well. FTTC is up to 20mb upload depending on the package you pick, which will allow your parents to upload those files faster meaning less time slowing you down.