Is it permissible to change a vow or revoke it before it takes effect and before the money is received, if the vowed amount is large and could affect one's future, knowing that the vow was made for the questioner's sister after a car accident?

Fulfilling a vow of obedience to Allah is obligatory, and it is not permissible to annul it except in case of inability, in which case an expiation for an oath (kaffarat yamin) becomes due. The impact of fulfilling the vow on one's future schedule does not constitute inability.