Can wills contain conditions which must be fulfilled in order for certain clauses to have effect? For example:
Uncle Fred wants his house to go to his niece, on the condition that the niece agree not to shack up there out of wedlock. What if, additionally, Uncle Fred stipulates that the house is to go to his nephew otherwise, including the case where the niece originally gets it and then violates the terms of the inheritance?
A husband leaves his inheritance to his brother, on the condition that his brother marry his widow and remain married to her for as long as they both should live. What if, additionally, the husband adds a clause stipulating that the estate is given to his children otherwise, including the case where the brother becomes divorced from the widow?
The creepy old lady down the street leaves a house to a group of friends, on the condition that they spend one night in it - and survive! Bwa ha ha ha
This question was inspired by this one on leaving a wife/girlfriend in your will.