Is it permissible to marry a younger man in an 'urfi marriage without documenting the marriage, in order to keep my pension, knowing that I will travel alone to complete the marriage and we will not live together in the same country?

The marriage is valid if the Shariah conditions are met, including the guardian (wali), even if it is not documented in court. The sister's marriage contract must be officiated by her guardian or someone authorized by him. It is not permissible for her to marry without a guardian, nor to travel without a mahram (definition). Likewise, she is not permitted to violate the condition set by the entity providing her deceased husband's salary if they stipulated that she must not remarry.