The specific gap between June and Jack — she was presented as his sister despite being nearly two decades older — is one of the details that, in retrospect, feels like a number that should have raised questions. But age gaps between siblings are not inherently suspicious. Families have children over long stretches of time. A first child in 1918 and a last child in 1937 would represent an unusually long span, but not an impossible one. The explanation would have been simply that Ethel May and John had spaced their children across many years, a narrative that required no active deception to maintain once Jack was old enough to notice things like ages.
Whether the specific details of the age gap were ever noticed or questioned by young Jack is not part of the available record. What is available is his own later statement, made after the discovery, that he had not suspected anything. He described the family he had known as his family. He described June as his sister. He described Ethel May as his mother. There is no account of him having had doubts that were later confirmed.
The household Jack grew up in was not a performance of a family. It was a family, constructed around a particular rearrangement of biological relationships that the adults had decided was the most manageable response to a difficult circumstance. The management of that rearrangement required the cooperation of June, who had to present herself as a sibling for the rest of her life; of Ethel May and John, who had to accept a biological grandchild as a legal and practical son; and of Lorraine, who had to carry the knowledge of what was actually happening in the house she grew up in.