Payments based on relationship can only be issued to minor dependent children, children under the age of 23 who are still in school, children who became completely disabled before the age of 18, or widows. A payment based on relationship likely could not be issued to you in this case.
Thank you all for your answers.
I wound up submitting a Form 21-601 including all the funeral expences, end of life assistance costs, and other medical bills I paid prior to my mother's passing. Both my sisters also submitted 21-601 forms even though neither sister paid any of the final costs. I had them fill out their own individual forms since the instructions included in the 21-601 specificaly stated that "each person claiming a share of the accrued benefits should file a seperate form 21-601" and also "The fact that a preferred beneficiary fails to file or prosecute a claim does not permit payment of his/her share of accrued benefits to a person or persons having an equal or lower preference".
This lead me to the conclusion that if neither of my sisters submits a claim, I would only be entitled to 1/3 of the accrued benefits since there are actually three persons entitled to benefits based on their relationship to the beneficiary. In order to receive all the accrued benefits, all three surviving children of the beneficiary would need to submit a Form 21-601.
Frankly, I'm a bit confused regarding the need to submit burial and final medical expences since paragraph C of the instructions for Form 21-601 states "If there are no living persons who are entitled on the basis of relationship, accrued benefits may be payable as reimbursement for last illness and burial expenses." This statement indicates that you only need to submit funeral and medical expences if you are not entitled to accrued benefits based on your relationship to the beneficiary.
Since according to the table included in paragraph C, my sisters and I are entitled to equal shares of the accrued benefits on the basis of "relationship" to the beneficiary, it would appear that we need no further justification to receive the accrued benefits.
I'll keep you posted on the final outcome.