The La Dola dagger has been the center of a similar event before.
In 'Bilingual bonus' on the main page, the Spanish teacher evidently mutters "This is weird. The dagger is Aztec, but it has a warning in Spanish". This, added to the fact that Ginny's father received the dagger in a display case that couldn't be opened, only smashed, points to someone adding the warning to the dagger more recently. In Aztec times, the dagger would only have been used by the priesthood, who would know all about the body-swap condition, and wouldn't need a warning. Given how language tends to shift and evolve, the Spanish teacher being able to read the inscription points to the last couple of centuries. Which means that someone used the dagger and encountered a situation similar to Millie's, and afterward someone connected with the incident carved the warning on the dagger in case it happened to someone else. Ginny's father is involved in the criminal underworld in some fashion.
The main evidence for this is the basement scene. The huge array of artwork in the basement would actually do credit to a small museum. But why have so much artwork that isn't on display? Unless it can't be on display for some reason, and judging by what we see of the house space isn't the issue. Implying that either Ginny's father is a collector who hoards stuff he doesn't want to display publicly - which would not be unusual, but then why not have the basement organized so that everything could be properly admired? - or having it readily viewable might get him in trouble. Valuable artwork has long been prized by the criminal underworld as a form of currency, and Ginny's dad storing stolen or even legitimately bought artwork that someone wants to keep their possession of quiet (say, to hide assets in a divorce) would explain quite a bit. The next Blumhouse slasher will be set in a The Truman Show homage
Where a slasher sneaks onto the set of the set. Alternatively, it will be a The Parent Trap homage
Where one of the twins turns out to be a killer or are stalked by a killer.