How to find Markov Blanket

In a Bayesian network, the Markov Blanket of a node is the set of the nodes comprises of the node’s parents, its children and its co-parents.

Definition: Co-parents of a node are the parents of its children.

Let us consider a Bayesian network as shown in Figure 1.

Let us now find out the Markov Blanket of X₄, the node highlighted in red in Figure 2.

Figure 3 shows the Markov Blanket of X4. X₁ and X2 are the parents of X4, X7 and X9 are the children of X4 while X6 is the co-parent of  X4.