# The slaves who have believing masters should not show them disrespect because they are brothers

Possible meanings are 1) it is because the believing masters are fellow believers that their slaves should not show them disrespect or 2) slaves should not think that because their masters are believers they can therefore show them disrespect.

# those who receive the benefit

"the masters whom the slaves help with their good work" 

# and beloved

"and the slaves should love them" or "whom God loves"