1 The death doctor tortured people during WWII
Before the movie events took place, a doctor, called the death doctor, made torture experiments on Jewish/polish people during WWII in the same house where the events now take place. The experiments were done in the basement of the house.
2 The spirits haunt the house
The spirits of the people who died in the experiments haunted the house as their bodies got buried in the basement. The spirit of Marcus, which is believed to be one of the victims of the death doctor, haunted the house as well.
3 The family broke Ouija's rules
The family broke Ouija's rules as they played in a graveyard, as the house had dead bodies buried in the basement, and because Doris played on her own which is also against Ouija's rules. This could be the reason why the spirits were released.
4 Alice believed she is contacting Roger
Alice, Doris and Lina believed they were contacting Roger, Alice's husband, but they were actually contacting the spirit of Marcus without knowing.
5 Marcus haunted Doris
Doris felt a pain in her neck. When she looked in the mirror, the spirit of Marcus haunted her. The Polish writings Doris kept in her room were actually part of Marcus' diary.
6 Roger's spirit gave Lina a hint
Roger's spirit, Lina's father, gave her a hint on how to stop the spirits. Lina realized she must seal the mouth of her haunted sister Doris to stop the spirit.
7 Doris haunted Lina
While Lina was sleeping, Doris whispered in her ears, thus haunting her as well. Lina still acted normally, but later on, the spirit will take control of her.
8 Lina sews Doris' mouth
Lina sewed Doris' mouth to stop the spirits just like her father told her. While Lina succeeded, the fact that she was haunted earlier forced her to kill her mother.
9 Lina was taken to a mental health institution
Lina was taken to a mental health institution and Doris' body was never found. Lina constructed her own Ouija bored and summoned the spirit of Doris.
10 Lina is the woman on the wheelchair
Lina grew up to become the women on the wheelchair, which was seen in part one of the movie. Lina was still evil and possessed and this is why she tricked her niece in part one of the movie to release the spirit of Doris.