4 ideas
The assembled relatives agree to shield her by staging the scene as a break-in by an outsider and coordinating their accounts for the police. Under investigation, the staged scenario and the confession itself come apart, and the premise that everyone was protecting the same truth turns out to be false.
When several people build one shared lie, each must improvise the details they alone handle, such as timings, footprints, and what they claim to have heard. The individual improvisations are never fully synchronized, so the lie leaks at the joins between them rather than at any single weak point. A coordinated cover-up is therefore more fragile than a solitary one, because its consistency depends on the least careful participant.
Once someone confesses, listeners stop asking who else could have acted and start asking only how to respond. This makes a confession a powerful tool for redirecting attention, whether or not it is true, because it replaces the investigation of the event with management of the confessor. Accepting a confession at face value hands control of the narrative to whoever produced it.
In a family bound by money and reputation, a cover-up can be read as a series of competing versions of events, each serving different stakes such as inheritance, the company's name, or one member's safety. Asking which member benefits from each version turns apparent loyalty into a readable map of interests. Seen this way, a unanimous agreement to protect one person can hide divergent motives among the people agreeing.