"Co-owner" is not a legal designation. She would have to be either a "tenant in common," or a "joint tenant with rights of survivorship
If she is the former, then she would have already owned one half of the property at your mother's death, and if your mother left her estate to both you and your sister 50/50, then you would only own 25% (1/2 of 1/2) of the property, because that is all your mother could transfer in her Will. However, as a 25% owner, you have the right to joint possession of the property, and that means that you don't need your sister's permission to move in. You can just move in, and if your sister won't give you the keys, then you can hire a locksmith to change the locks.
But, you can't keep your sister out, either.
If your sister is a "joint tenant w/ RS," then she owns the property outright as her sole and separate property, and there is absolutely nothing that you can legally do to reverse that situation -- which also means you have no right to possession of the property -- period.
So, you may want to get a copy of the deed from the county clerk, and then let me know what it says. That, combined with what your mother's Will states, determines your rights.
Hope this helps and hapy new year (assuming that's possible under the cirucmstances).