Is it permissible for me to take a quarter of my intellectually disabled brother's monthly income as a salary for my care of him, and does this affect my reward from Allah?

If you are poor, you may take from your brother's money an amount less than what suffices your needs or the equivalent of your wages for managing his affairs, according to the Almighty's saying: {And whoever is rich, let him abstain [from taking]; and whoever is poor, let him take in a reasonable manner.} (Quran 4:6). However, if you are rich, you have no right to take anything from his money for yourself, unless the judge stipulates something for you in return for your engagement. Taking wages does not nullify the reward, but it diminishes it.