Since you have to outlay money on a secondhand Mac, and get an IT guy to set it up for you, why not buy a set of these, as I suggested? They are pre configured out of the factory for you. Cheaper than a secondhand Mac.
Your sister can plug in the home proxy dongle, no need for the IT guy. The set comes with two power supplies and two short ethernet cables. The companion dongle can be posted or couriered to you. Plug it in at your house in Thailand, it then connects to the one at your sister's house, you connect to the one in your house via WiFi, and you are on your sister's IP Address for every website you visit, including streaming websites.
They are small and use no electricity and are left powered on all the time. There's no ongoing subscription fees if you buy a set and don't go through their app and server.
I have a set of these and they work well. I had untold trouble streaming Foxtel in Thailand. I tried many things, including multiple VPN's. These get around all the geo blocked sites I use.
https://www.homingsystems.com
Here's an old youtube video about it. These are the older models but they still do the same thing.