The brother recognises his duty, and prepares for vengeance. The injured one probably desires that she may be left to fight her own little battles alone.
"Then, by heaven, he shall answer it to me," Sir Felix had said very grandly, when his sister had told him that she was engaged to a man who was, as he thought he knew, engaged also to marry another woman. Here, no doubt, was gross ill-usage, and opportunity at any rate for threats. 
Sir Felix had vowed to avenge his sister for her ill-usage by Paul Montague. 