he was too cowardly to tell her the truth
Why is courage regularly confused with headlessness?
MC is not headless, and he was quite reasonably afraid of losing his sister, who, at that moment of memory loss, might not understand correctly such a joke as a child from an incestuous relationship with a sibling.
Apparently, the days when Katherine "processed" her, helping her decide on sex with her brother, fell out of her memory.
No, anything but such stupidity as giving shocking information to a pregnant girl injured in an accident.
What if she miscarries from shock?
if you want you can call me or even MC a coward - I don't care. I myself would endure such a nickname for the sake of my sister. I couldn't bear the loss of my sister and/or child.
and the sister could take and go to live with the parents of the "father of the child" and forbid the parents to let the brother in on the threshold, considering him a liar who wants to discredit the name of a loved one.
No. MC did everything right.
he acted as the mother did with the child from the parable of King Solomon, when two women, looking identical, both declared that the child was theirs. and then Solomon offered to cut the child and one said, "it's not for me so let no one get it, cut it!" the other said "no, let her be the mother, don't touch the baby!" the real mother agreed to lose the child in order for the child to live, even if he would live with a false mother.