Power System Analysis - YBus Matrix
The common systematic nodal voltage analysis Y-matrix hasn't such term as y12'. Where it comes from - that's possible only to guess. I guess an error. The solution has been written for different network. There a distributed capacitance of a long line is approximately divided equally to nodes 1 and 2.

Actually that can be also a drawing convention, the distributed capacitance is drawn at one end, but there's a rule how it should be taken into the account in calculations. Unfortunately I haven't your book. 

If we assume the drawing covention case to be true, then capacitive impedance -10j which has been drawn between 2 and gnd, is actually distributed along the cable and calculated approximately as divided equally to nodes 1 and 2.