why was Ms Stapleton so eager that sir Henry should return to london

I heard the sound of steps and, turning round, found a woman near me upon the path. She had come from the direction in which the plume of smoke indicated the position of Merripit House, but the dip of the moor had hid her until she was quite close.
I could not doubt that this was the Miss Stapleton of whom I had been told, since ladies of any sort must be few upon the moor, and I remembered that I had heard someone describe her as being a beauty. The woman who approached me was certainly that, and of a most uncommon type. There could not have been a greater contrast between brother and sister, for Stapleton was neutral tinted, with light hair and gray eyes, while she was darker than any brunette whom I have seen in England–slim, elegant, and tall. She had a proud, finely cut face, so regular that it might have seemed impassive were it not for the sensitive mouth and the beautiful dark, eager eyes. With her perfect figure and elegant dress she was, indeed, a strange apparition upon a lonely moorland path. Her eyes were on her brother as I turned, and then she quickened her pace towards me. I had raised my hat and was about to make some explanatory remark when her own words turned all my thoughts into a new channel.
“Go back!” she said. “Go straight back to London, instantly.”I could only stare at her in stupid surprise. Her eyes blazed at me, and she tapped the ground impatiently with her foot.
“Why should I go back?” I asked.
“I cannot explain.” She spoke in a low, eager voice, with a curious lisp in her utterance. “But for God’s sake do what I ask you. Go back and never set foot upon the moor again.”
“But I have only just come.”
“Man, man!” she cried. “Can you not tell when a warning is for your own good? Go back to London! Start to-night! Get away from this place at all costs! Hush, my brother is coming! Not a word of what I have said. Would you mind getting that orchid for me among the mare’s-tails yonder? We are very rich in orchids on the moor, though, of course, you are rather late to see the