Estate Law Questions? Ask an Estate Lawyer.
It really depends on what the specific terms of the trust are (you are probably going to need to speak with a local trusts and estates attorney).
However, in general, your brother's heirs are going to be entitled to his share of the trust assets.
Just because his heirs were not identified as separate beneficiaries does not mean that his estate forfeits his share of the trust.
Trusts do not go through probate. (See: http://www.nolo.com/legal-encyclopedia/texas-avoiding-probate-31945.html).