Would the brother be sinful for offering water and tea to construction workers who are not fasting in Ramadan, knowing that the materials used would spoil if the work were delayed, and he provides these to them at the time of the Fajr athan and leaves them in the house?

What workers do by deliberately breaking their fast in Ramadan without an excuse is a great evil. Work is not a permissible excuse for breaking the fast, even for those performing arduous tasks. Your brother must not help them break their fast, as that would be cooperation in sin and aggression. Rather, he should advise them and explain the seriousness of their actions.