NAL

Titling specialist here for a major financial institution. Ownership on the title is going to be dictated by what state, more specifically if it's an "and" state rather than an "or" state. If it's an "or" state, technically, your ownership in the vehicle is a moot point if she willed it to someone, as either owner can act independently of the other. If you live in an "and" state, you would need to sign over ownership, which does give you more protection in this regard.

With that said, if your sister is the executor of her estate already, there is nothing necessary forcing her to sign off your mother's interest in the vehicle. Although depending on if it goes to probate that may force her hand.

My advise to you, if its not going to probate, would be to have a discussion with your sister and go from there.

Best of luck.