None of the things above are clear for the great majority of the film, and the fact that they don't matter is a key element of the film: How important are blood relations vs. true compassion and love? The answer is... not, in each lived life.
This is what I get:
The mother figure killed her husband and was not prosecuted due to self defense, which is revealed in a discussion with the prosecutor in the film.
The father figure in the film was somehow involved in the killing. He spent time in prison, though it is not clear whether it was related to this crime or not. It is not clear whether they were lovers at the time.
The grandmother figure is not related to either, but welcomes the two into her home as family despite no blood ties, it seems.
The other adult female seems to be the granddaughter of the grandmother figure's ex husband, but with no blood relation to the grandmother figure.
The male child was abducted from a car in the parking lot of a pachinko parlor (a gambling establishment like the one the grandmother figure is shown at). The father doesn't convincingly answer the male child's question as to whether he was abducted merely as part of a car robbery or purely out of charity.
The female child's story is shown in its entirety.