Is the marriage valid if the woman—who has been divorced twice and has two children—agreed on the condition that the husband would not be responsible for expenses, and she did not inform her guardian of this condition for fear of rejection? And if she informed him and he refused, can her brother be her guardian without authorization from their father?

If the conditions for a valid marriage are met, such as the guardian's permission and the presence of witnesses, then your marriage is valid. The guardian's lack of knowledge about the condition to waive maintenance does not affect its validity, because this condition is void, and the wife is entitled to maintenance.

Her brother is not permitted to marry her off while her father is alive, because it is not permissible for a more distant relative to give her in marriage when a closer relative is available, unless there is a legitimate Sharia (definition) reason, such as the father preventing her marriage. However, the father's rejection of the condition does not justify the transfer of guardianship to the brother.

This condition is as good as nonexistent, so there is no need to mention it. After the marriage is complete, the wife may waive her right to maintenance.

Marriage may open doors to provision, as Allah Almighty says: "And marry the unmarried among you and the righteous among your male and female slaves. If they should be poor, Allah will enrich them from His bounty, and Allah is all-Encompassing and Knowing." (Quran 24:32). And as stated in the Hadith (definition): "Three are rightfully entitled to Allah's help: the Mujahid (fighter) in the way of Allah, the Mukatab (slave who has a contract of emancipation) who intends to pay, and the one who marries seeking chastity."