Y
ou don't state whether Telkom or MWeb controls the ADSL line.
If Telkom, you should be able to sign up for various free 1GB accounts, which you can compare against MWeb's (not so sure if MWeb controls the line, whether you can sign-up and test other ISPs ; others on this forum may know).
eg.
http://www.afrihost.com/landing/freegb/
Axxess also have free 1GB / month (sister-co of Afrihost), so do / did Web Africa, Gridhost, Vox - try some or all out. Note that some of them may require you putting in Credit Card details as well.
After comparing, your family should be able to determine if it's MWeb or the line itself (assuming all of them are terrible & have issues). A good test is to stream some YouTube vids ; if the progress bar moves slowly or fast on eg 480p or 720p, to me that's a good indication of how the ISP's performing - case in point - my Vox connection doesn't always do so well, but downloading I get full line speed speeds through them, so I think they're certainly controlling aspects of the usage.