That will depend on 2 things... how big the tiger barb is at this point (they get quite large) and what else is in your brother's tank.
Whenever a customer asked for a fish to put into someone else's tank, we warned constantly about making sure that the other person's tank is suitable, both in water quality and compatibility. If you can get a stock list of what is in your brother's tank, we can then help you further.
The minimum tank size to raise a tiger barb by itself should be 10 gallons. If the barb is very small (under 2 inches), then you could safely keep it alone in a 2 gallon for a short period of time, provided it has the right equipment hooked up, including a filter and heater, decorations, and regular weekly water changes of 20%.