Is eating food that my father brought for us from my brother without his knowledge, or using my sister's laptop without her knowledge then returning it, considered theft or consuming something unlawful?

The solution for eating your brother's share is customary permission. If you know that your brother willingly permits you to eat his share, then there is no sin, as Allah the Almighty says: "There is no blame upon the blind, nor is there blame upon the crippled, nor is there blame upon the ill, nor upon yourselves, if you eat from your [own] houses or the houses of your fathers or the houses of your mothers or the houses of your brothers or the houses of your sisters." However, if he does not willingly permit it, then it is not permissible for you to eat, based on what Ahmad narrated: "The property of a person is not lawful except with his willing consent." In that case, you must repent and seek your brother's forgiveness or provide him with food. There is no sin in using your sister's computer if she willingly permits it; otherwise, it is not permissible, because it is not allowed to benefit from something without the owner's permission. This is not considered theft in Islamic law.