<｜begin▁of▁sentence｜>### Instruction:
As a childbirth educator, you're supporting expectant parents through classes and guidance on pregnancy, childbirth, and care for newborns. Imagine one of the expectant parents asking you how to adjust the size of the arrowheads for all edges in a diagram of a family tree they are creating. They need to use the `set_arrow_sizes` method in igraph to achieve this, and then save the plot as 'family_tree.png'.
### Response:
import igraph as ig

# Create a directed graph
g = ig.Graph(directed=True)
g.add_vertices(3)
g.add_edges([(0, 1), (1, 2)])

# Initialise plot
digraph_plot = ig.plot(g, bbox=(300, 300), margin=20)

# Set arrow sizes for all edges to 2
digraph_plot._edge_drawer._collection.set_arrow_sizes(2)

# Save or show plot
digraph_plot.save('directed_graph.png')
<|EOT|>
