There are three people involved in this scenario.
- Jessie Clark
- I (the main character, the speaker), let's call her Jane
- The Brother (of the speaker), let's call him Jack
Location: Jessie's garden.
- What is the subject doing?
Shes doing a show, of some sort.
- What is Jessie doing?
That is unspecified, let's assume he's just standing there, watching.
- What is Jack (the brother) doing?
He is digging on the ground beside a tree/ or on the tree itself, which stands behind Jessie.
So, to sum up,
Jessie Clark has a back garden. Jane is doing some sort of a show, while her brother, Jack, is digging up something beside/ on a tree, right behind Jessie.
Other possible interpretation is that the 'him' referred to at the end of the sentence is addressed as the brother.
But then, how does a kid dig at a tree behind himself?
Possible Interpretation by V.V:
The brother is digging (on the ground) by a tree, which happens to be behind him.
(This is the very valid interpretation)