Mr has son from previous marriage
Mrs has daughter from previous marriage
Mr & Mrs have no children together
Commonly in a Will, Mr & Mrs leave everything to each other and then 50/50 to children on the second death.
This can be problematic for example, Mrs dies then Mr will inherit everything. Should he re-marries or change his Will, when he dies his whole estate can pass to his new wife and eventually on to her children, when she dies. This would mean that the original children get nothing and is called Sideways Disinheritance. Make sure if you have children from different relationships your wills are watertight and your wishes are understood and catered for correctly. Always tell your Will writing professional about your full circumstances.