Ryse can be a little bit CPU heavy, and is also one of the few games where RAM speed seems to make a big difference, at least on Intel platforms. You may not get the same results as your cousin if he has higher end parts backing the R9 390x compared to what you get, so it might help to know the full specs of his system.
I got good results in Ryse with a GTX 970 with my setup you can see in my signature. Generally speaking, you're probably going to want a Core i5 or Core i7 CPU, and you're going to want faster RAM. If you get an older Haswell platform with DDR3 RAM, then you may want to get DDR3-1866 sticks or better, since that particular game is sensitive to RAM speed. If you get the newer Skylake or go Haswell-E, and are getting...