What Do You Think

You are writing about the plot of Mary Stewart's novel Thornyhold.  I had to Google some other book reviews to understand what you are trying to stay.

Gilly's cousin Geillis visited her occasionally when Gilly was a child.  Your sentence makes it sound as if it was a single visit.  Geillis did remind Gilly of a fairy godmother, but your sentence is too vague on this point.

Given the fact that Gilly lived with two parents who had a relatively happy marriage, very desolate sounds too strong to describe her childhood.

I suggest the following revision:

Gilly had a lonely childhood, except for the occasional visits of her mother's cousin Geillis.  Geillis seemed like a fairy godmother to her.