If Catherine is so conscientious as to refuse Henry because accepting would perpetuate the rift in his family, surely she should break off the engagement on the same principle as soon as he explains the situation? The only thing that's changed in the meantime is that she's officially let the cat out of the bag and revealed her own inclination, without reference to the rift. So it seems to me that the critical factor - the difference between rejecting at one point or breaking off a few minutes later - is that by rejecting Henry's offer she could have kept the cat in the bag and denied her feelings, which would've been tragic.