I wanted to rethink the plan of whole flat considering the position of the sun. First of all each and every space reaches its own daylight and there will be no dark entrance As I got it from the pictures, north view of the flat is more central. It is a good idea to locate the living room to north. Because living room and kitchen is a whole space, they have two different facade and enough daylight.
So master bedroom can be at south with more daylight. I thought morning sun is important for a bedroom. Moreover while back balcony is private, front balcony will be used by guests.
I proposed a guest room for your nephews but it can turn into a hobby room with other furniture. In any case there is a huge storage in that room. There is one more storage in entrance area. You may cancel one of them if you like. It is also a good idea to have a console and mirror instead of this second cabinet.