How can I get my father to be my son's brother-in-law and that I am my brother's grandfather?
For this to be possible I need to marry a widow who has an adult daughter. My father marries that daughter, who is my stepdaughter. So my wife is the mother-in-law of my father and my father is his stepson. Then my stepdaughter has a son who will be my brother (my father's son), and also my grandson, because he is the son of my wife's daughter. Therefore I am also my brother's grandfather. My wife had a son with me. So it turns out that my mother-in-law (my stepdaughter) is my son's sister and also a grandmother, because she is her stepson's son (who is me). As my son's sister is married to my father, my father is my son's brother-in-law.