It is assumed that his elder brother and sister-in-law did it

And it is poisoned~

Because in the old house, except for his father, it is his elder brother and sister did not get poisoned

And his elder brother and sister-in-law did not have any children

Maybe because of jealousy that Liu did rich children

Maybe in order to drive them out of the old house to take over the house

All in all, this explanation is the most reasonable and reasonable. I think this explanation is the most reasonable~