Does a girl's silence about her sister's husband cheating by drawing more electricity than agreed upon from a neighbor's generator invalidate her fast?

Your family and your sister's husband are not permitted to violate the agreement made with the neighbors, based on the generality of Allah Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (PBUH) saying: "Muslims are bound by their conditions in what is permissible." What they have done is unjustly consuming people's wealth and is deception. The sin is magnified because the harmed parties are the neighbors.

We advise you to seek the mediation of good people to explain the legal ruling to them. If they accept and pay the value of the electricity they consumed in excess of their share, then praise be to Allah. Otherwise, instruct the person in charge of the generator about what is happening so that he can hold them to the agreement and fine them for the excess amount. As for your fasting, it is not affected by their actions.