Had it not occurred to her that her own interests were involved she would have said nothing on the subject; but she feared her brother,--feared even his misconstruction of her motives, even though she was willing to sacrifice so much on his behalf,--and therefore she resolved to tell him all that she knew. 
If she was silent, she feared her brother might misconstrue her motives, even though she was willing to sacrifice so much on his behalf; and therefore she resolved to tell him all that she knew.
