This is my theory
Thi's mom planned to kill Siya becaue she knew about the affair (but not the whole picture of the relationship) that way if Siya dies, then Pipop has no one else to really split money to. She gives it enough time for Pipob to change the will. She knows it is done and then has Pipob killed. Reason?
1. She wants Pawanee to know the feeling of losing her son because she had to leave her son, once upon a time. She didn't anticipate the will, but now that Thi is legally wedded too Siya, she is going to start trying to kill Siya off, so her son gets everything.
At least, that is one of my theories lol.