Why did Sir Henry think Mr. Stapleton appeared to be mad?

On Page 143, we learn that Sir Henry is interested in marrying Stapleton's sister, which isn't welcomed by her brother (we later learn that she is his wife). Watsom inquires if Sir Henry had ever seen evidence of Stapleton being crazy before.

‘Did he ever strike you as being crazy—this brother of hers?’

‘I can’t say that he ever did.’

‘I dare say not. I always thought him sane enough until to-day, but you can take it from me that either he or I ought to be in a strait-jacket. What’s the matter with me, anyhow? You’ve lived near me for some weeks, Watson. Tell me straight, now! Is there anything that would prevent me from making a good husband to a woman that I loved?’