You're right, its 1/3 and 2/3. I wrongly split D's share equally between B and C.

I'd propose removing the figure and using the following text (which is close to your text):

For example, assume streams A and B share a dependency, and streams C and D
both depend on stream A.  Prior to the removal of stream A, if streams A and D
are unable to proceed, then stream C receives all the resources dedicated to
stream A.  If stream A is removed from the tree, the weight of stream A is
divided equally between streams C and D. With stream D still unable to proceed,
this results in stream C receiving a reduced proportion of resources (one
third, rather than one half).