I can’t envy these people.
The situation they have to face up to is not easy at all. Alison is in denial, but Jeff convinces that they need to know. The first reason is that Katherine might have that mutant gene that can develop into some kind of cancer. When they get tested, they get confirmation that Katherine is not theirs. They have to tell the girl because of the cancer risk, and Katherine reacts strongly, refusing to meet his biological family. I imagine that sooner or later this reluctance will fizzle out and she will want to know where she comes from. I am surprised that Jeff and Alison have not even mentioned that their real daughter is with the other family. Aren’t they curious about her? Don’t they want to see her? I imagine that at some point they will wonder and want to know. The situation isn’t easy at all.
In the parts when Alison reminisces her life thirty years ago, we read about her working hard for her A levels. She even takes the first exam which is maths. She is determined to go to university and leave the poverty she has to live with every day. Yet, during the exam a thought comes unravelled: she hasn’t had her period for two months. So she skips the next exam to buy a pregnancy test, which confirms that she is pregnant. What does this mean? Did she have an abortion thirty years ago? Or did she give the baby away?