The most basic structure of this sentence is the following
The true motive of her story is hard to know.
The true motive of her story is hard to know.
The prepositional phrase
of her story
of her story
is rearranged and placed weirdly. This is not grammatical.
The sentence structure is also changed and would read without the prep phrase like the following
It is hard to know the true motive.
It is hard to know the true motive.
In this case an implied subject "you" is used. When you include the implied subject
It is hard for you to know the true motive.
It is hard for you to know the true motive.
Hopefully it will be easier for you to analyze the grammar.
Authors of fiction novels like "A Little Rose" often use improper English to add style to the writing. If the author wishes to portray a rural character with no schooling, they may use improper English to reflect this without stating it outright.
Notice that Charlotte Brown has made this stylistic change at the cost of clarity. You should always strive for clarity.