The answer to this question would be Rh incompatibility.
Rh or rhesus is a kind of antigen that can be found in human. Rh negative means that the mother doesn't have Rh antigen in her cell, so Rh antigen might be considered as "foreign body" and might activate white blood cell upon contact. If the father of the child has Rh positive gene, the child might become Rh positive and the child antigen can activate immune reaction and eliminated by mother white blood cells.
Rh incompatibility should be more severe in the second child since the mother white blood cells are more prepared than the first child(already sensitized).