Are the leaves attached to strawberries poisonous?

This is not true. About the only commonly consumed vegetable I can think of that has poisonous leaves would be Rhubarb. The leaves contain a strong dose of Oxalic Acid, which can be toxic if consumed. There are also a few wild herbs and vegetation that have toxic substances in them. But, I seriously doubt if your brother is going to be dining on a Poison Ivy Salad anytime soon. At any rate, the leaves of strawberries are harmless, and just another vegetable.