The treaty you refer to was between the US and Britain, not Canada, however I am well aware that the US takes the position that Aboriginal people from Canada with “50% American Indian blood” may live and work in the United States without restriction. However, the entitlement to US social security benefits is not specifically effected by the treaty.
Under US social security laws, a person must either be a citizen or a permanent resident of the United States to be eligible to collect benefits. Because only foreign nationals who are permanent residents of the US are eligible to collect social security benefits, and given the fact that you are Canada born aboriginals---while the treaty allows you to live and work in the US, it does not specifically confer permanent residency on you and your sister.
Because your sister was assigned her social security card so many years ago, it was not necessary that she be a permanent, legal resident of the US when she worked and paid into social security. However, she must now apply for a green card before she will be eligible to collect her social security benefits.
I understand that you may be disappointed by the Answer you received, as it was not particularly favorable to your situation. Had I been able to provide an Answer which might have given you a successful legal outcome, it would have been my pleasure to do so.
If you have additional questions, you may of course reply back to me and I will be happy to continue to assist you further until your questions have been answered to your satisfaction.
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Thank you in advance. I wish you the best in your future,