What explanation did Stapleton offer for his enraged behaviour to Sir Henry upon discovering the latter with his sister ?

From the text:

"His sister is everything in his life, he says. That is natural enough, and I am glad that he should understand her value. They have always been together, and according to his account he has been a very lonely man with only her as a companion, so that the thought of losing her was really terrible to him. He had not understood, he said, that I was becoming attached to her, but when he saw with his own eyes that it was really so, and that she might be taken away from him, it gave him such a shock that for a time he was not responsible for what he said or did. He was very sorry for all that had passed, and he recognized how foolish and how selfish it was that he should imagine that he could hold a beautiful woman like his sister to himself for her whole life. If she had to leave him he had rather it was to a neighbour like myself than to anyone else. But in any case it was a blow to him and it would take him some time before he could prepare himself to meet it. He would withdraw all opposition upon his part if I would promise for three months to let the matter rest and to be content with cultivating the lady's friendship during that time without claiming her love. This I promised, and so the matter rests."