Is it a sin to split expenses fifty-fifty with a brother, despite the questioner owning more electrical appliances and consuming more electricity, knowing that this was a condition set by the questioner before they shared accommodation and that the brother continuously objects to it?

The fundamental principle is that each partner should have a separate water and electricity meter to avoid one person paying more than their consumption, as an agreement to divide bills contains an element of gharar (uncertainty/deception). If it is genuinely impossible to separate the meters for reasons acceptable under Islamic law, then the bills must be divided according to each party's consumption, unless the one with less consumption agrees to an equal division. If the less-consuming party's acceptance of an equal division comes out of compulsion, then it is not permissible to divide expenses equally with them. Instead, the bills must be divided according to actual consumption, and one must seek their forgiveness for what has passed, or return what was unjustly taken from them.