Ouija boards to my knowledge are different from and not inherently connected to Voodoo. They actually originated as a Parker Bros (now Hasbro) parlor / board game and did not even have a spiritualist or supernatural connotation until they were appropriated by spiritualists and mediums, post-introduction.

Ouija boards are functionally explained by the ideomotor effect. Your sister's trance would be a manifestation of suggestible hysteria. Ouija boards are often used in dark / spooky surroundings with a lot of suggestions and atmosphere so a young suggestible child intimidated or frightened by such a melieu could certainly respond in the way you describe. Hasbro recommends the game for children over 8, and I'd say, over 10 would be even better, since that's the age at which children start to get a better grip on distinguishing the real from the imaginary.

Voodoo is something I'm not terribly familiar with but perhaps you grew up in an area where voodoo lore was prevalent and that was why the owner of the ouija board was mixing in voodoo semantics or something, or why the two are connected in your mind.

Another source of that notion can be Christian fundamentalism, which tends to lump Ouija boards, witchcraft, voodoo and anything else remotely woo-ish but not aligned with their doctrines, together as "demonic" or "Satanic".

There's no such thing as magic.
Ghosts do not exist.
There are no demons trying to possess humans.
That should cover it, but there's no Santa, Easter Bunny, or Great Pumpkin either.

As so many have mentioned, there is no evidence that ouija boards can do anything other than pass the time. But it's possible if your sister believed it was real that something physical did happen to her. The mind is incredibly powerful. When people believe something it becomes real for them. An example (which actually was copied in an episode of MASH) -- In the Korean War, morphine sometimes ran out. When that happened, docs told patients they were getting morphine but used saline solution (salt water) instead. The mere fact of believing that the injection contained real medicine resulted in a lot of patients reporting less pain. It's called the placebo effect. It's super powerful. It's the same thing you see in some revival meetings when people can suddenly walk. There was probably no medical basis whatsoever to their 'disability'. When someone they trusted to be able to cure them told them they could walk -- they could. But let me hasten to add that most of the time these 'cures' were just shills (actors working for the 'house'. There was never anything wrong with them at all.

Voodoo relies on its victims believing in it.

A friend and I once got a pos potential child predator to move because HE believed in voodoo

About as real as jesus, astrology or any other nonsense

The correct term is a Ouija Board or spirit board, it is nothing to do with Voodoo, which is a religion like any other and like all other religion relies on trickery, psychology, fear and superstition but this case with a dash of pharmacy made from naturally occurring hallucinogenic plants thrown for good measure.
Ouija was invented by Hasbro Gaming in 1890 as a game (it even came with rules and you could win a game of ouija) by their founder Elijah Bond, though so called talking boards and prophesy board had existed since the 12th century. The idea of using them to contact the dead was a later addition and only became popular during world war 1. It was when a Ouija board was used in the film the exorcist in 1973 that this use became popular as the sole use for the boards.
They basically induce fear and expectation and get "messages" from the sub conscious minds of the players. Which is why they some times seem true.
Most countries have their own variation on this meant to frighten children in Japan for instance it is called a Kokkuri san cloth, meant to summon a fox spirit to pass messages from the other side.