Your internet speed (upload/download) really depends on:

Your Network Card
Your Line Speed (max available from your ISP)

Its possible that you have a much better network card.

Another possibility is that your connection is 'hogging' all of the bandwidth (upload/download speed).

The test would be to disconnect your router / turn it off and see if your mother-in-law's speed improves. If it does then the router (or more likely a setting on the router) is the problem.

If the problem still exists after your wireless connection is taken out of the equation then something else must be considered.

It could be possible that your mother-in-law has some sort of virus or malware infection. Some infections use your internet connection to transmit information to other computers. This can slow down the response time of your computer and your internet connection.

The other possible cause is the way you've set up the router hardware. Although I'm not sure how that would work or really affect things...I assume you've just split the upload/download coaxial cables into two?

A quick test would be to swap around the coaxial cables eg the two that go to your router, put them into your auntie's modem and vice versa.

If you have done that

First step though would be to disconnect the wireless router.

Do a test at Speedtest.net - The Global Broadband Speed Test before and after to see if there is any change.