If your sisters are just going for Minecraft do an AMD A-series with integrated graphics. AMD Integrated graphics are pretty good, certainly enough for Minecraft. It usually is somewhat cheaper to build your own computer and it gives you much greater reliability in terms of mobo + PSU especially. I would just go with an A8 or A6, a Richland A4 or Athlon 5350 if you are really on a budget. Get a motherboard, 2x4GB RAM and a 120GB SSD and a nice case and decent PSU and you're set. You can buy a Windows license or install Linux, Minecraft actually works well on Linux. If you are using some distros you can barely tell the difference from a GUI user perspective. But I wouldn't bother with a discrete card. They are expensive, more difficult, and not needed for Minecraft.