What is it about Mrs. Sappleton's niece that causes Framton additional distress?

In the story, Mrs. Sappleton's niece, Vera, accompanies her story with acutely expressive facial and bodily expressions. It is these dramatic theatrics that cause Framton additional distress.
After telling Framton the Gothic story of how Mrs. Sappleton's husband and two brothers died, Vera shudders noticeably. Her dramatic action adds to the eerie nature of the story; being of a gullible nature, Framton becomes convinced of the truth of what he's hearing. He begins to see Mrs. Sappleton in a new light and thinks she is deluded. Poor Framton is thoroughly terrified but still remains seated, possibly because of his ingrained social training.
While Mrs. Sappleton continues talking about welcoming her husband and brothers, Vera further distresses Framton by resorting to more dramatics.

Framton shivered slightly and turned towards the niece with a look intended to convey sympathetic comprehension. The child was staring out through the open window with a dazed horror in her eyes. In a chill shock of nameless fear Framton swung round in his seat and looked in the same direction.

When the men do appear, poor Framton is terrified beyond endurance, and he flees the scene altogether. So, Vera uses Framton's trusting and gullible nature against him. By resorting to dramatic expressions, she is able to imbue her story with spine-chilling authenticity, an accomplishment that later causes Framton extreme distress.