Are there additional entry requirements for Canadians returning overland given the Covid-19 pandemic?

As a Canadian citizen, you are entitled to enter Canada and do not need to obtain any permit. However, there are at least three problems with your plan:

• Your refusal to get vaccinated puts you, your family, and the communities you will travel through and stay in on both sides of the border at unnecessary risk. Many people, not least including many in Canada, would love the opportunity (that you are wasting) to reduce this risk. You don't give your own age, but given your mother's age, I would not be surprised to learn that you yourself are at high risk for bad complications were you to come down with covid-19.
• All people entering Canada (with some exceptions that do not apply here) must quarantine for 14 days upon arrival. Note that you may not quarantine (in addition to other restrictions) in a place where you will have contact with people who are 65 or older, so this would rule out, for example, staying at you mother's house, and possibly your sister's, depending on her age.
• Depending on the type of tests available in Great Falls, it may be unlikely that the results are available quickly enough for you to be tested for covid-19 on your way north without an extended stopover. The testing requirements for travelers driving to Canada specify only that a test must be obtained 72 hours or less before arrival, so depending on how long you will take to drive and how long the results from the test you can get will take to be processed, you may wish to obtain a test at home or earlier on your journey.

More generally, review the requirements for driving to Canada from the Canadian government and make sure that you comply with them, including any that I didn't mention here. The situation is dynamic and could change on short notice, so make sure to review any updated requirements close to your planned trip.

As a Canadian citizen, you are allowed to enter Canada for any reason, no matter if you are entering by land or air. However, like everyone else entering Canada, you are subject to quarantine and COVID-19 test requirements. As I understand it, you have to provide a COVID-19 test from within 72 hours of your entry to Canada, and be subject to a mandatory 14-day quarantine after entry (even if your test is negative).