Molecular biology deals with the structure and functions of biomolecules. On the other hand, systems biology is the study of organism in a extreme crossdiscplinary perspectives where the biological phenomenon are studies on the basic of computational biology, modelling, methamatics, computerscience etc.
Systems biology is the conceptual and mathematical modelling of complex biological systems.It is a biology - based interdisciplinary field of study that focuses on complex interactions within biological system, using a holistic approach ( holism instead of the more traditional reductionism ) to biological research. While genetic engineering allow research using molecular biology concept. For more details consult https://www.ncbi.nlm.nih.gov