Is it permissible for a brother to take money designated as charity for a needy Syrian family and spend it on himself, claiming to be Syrian, even though he is not needy and is employed?

Your brother must adhere to what he was entrusted with: delivering the trust to those specified by the donor. He is not permitted to take anything for himself except with the donor's permission. If he does so, he has betrayed the trust, and he must repent to Allah the Almighty and guarantee the money he took for himself, returning it to be spent on the Syrian family, unless the donor agrees after being informed of the truth.