Consider the events in Chapter 7 when the reader is first introduced to Jack Stapleton as a character. First of all, he himself declares that he is the only person to know the secret paths that penetrate Grimpen Moor, the terrible swamp that is so dangerous to both man and beast. Secondly, his sister delivers a warning to Dr Watson that seems very strange and intense, whilst her brother is away, so that the warning is delivered in secret, without her brother knowing. Note what she says:
"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 beauties of the place."
She only feels able to deliver this warning when her brother is not there, and she is very quick to change the conversation as she sees her brother returning. What is even stranger is the way in which she then goes on to retract that warning as Dr Watson leaves, strangely ignoring what she said to him before, perhaps suggesting that she is being forced into doing and saying things by her brother. These facts all point to Jack Stapleton being a very strong suspect indeed.