Is it permissible to bear witness to an ‘urfi marriage with the intention of making lawful the deceased mother-in-law’s pension, knowing that the husband cannot improve his income?

What your brother wants — to divorce his wife and then marry her عرفيًا (customarily) without documenting the contract, in order to obtain his deceased mother-in-law's pension — is forbidden deception (ghish muharram), and it is not permissible for his wife to take this money. Your witnessing of the customary marriage contract is originally permissible if it fulfills its legal conditions, and it is not considered aiding in deception. However, you must forbid your brother from this deception and explain its ruling to him.