Well the obvious plan is revealed, as well as a partial justification: her mother wants another child but medically this may be difficult. That all was pretty easy to pick up. But there's still a lot we don't know; how did they find him, why does the mother want another child so badly, what sort of medical issue is she dealing with, is the end goal a family or for them to get a child and then leave without a trace...
There's a lot to know. But this is a significant step because the plan has been revealed to him. It's particularly interesting because it comes while he's still with his mom and sister, so there's room for him to turn back. He can realize that they were right, this isn't the horror ending where after he's moved in with them in a small cabin far away without internet or phone access they spring their plan on him. It's a heavy story beat that landed while he still has a lot of agency.