If your parents are 1/3 to 1/2 of a mile away from the closest box with those speeds, then it's likely the 'box' is a crossbox (wiring junction) instead of a DSLAM (networking card). Where the DSLAM is that is feeding that crossbox... well that could be much further.

If the loop length is .34-.5 miles (which is quite good) with no impedance mismatch or bridge taps or other various copper faults, then you would/should be able to sustain 10+ Mbps on an ADSL2+ connection.

My guess is that the distance is further than it seems, and the line may not have been reconditioned from phone to internet purposes.

Anyway I bring it up because distance is the biggest determinant of bandwidth when it comes to copper, and if the distance is great then there is little that fixing copper faults can do to improve internet speeds at your parents' place.