We never actually meet Mary Finch, Isabel and Ruth's previous owner, because we kind of go to her funeral on the first page of the book. Still, we feel her influence throughout the book, especially when Isabel finally decides to escape.
To begin with, Miss Finch freed Isabel and Ruth in her will. Her brother, Robert, may staunchly refuse to face this fact, but she still did it. There's no reason why Momma would give her daughters false hope by lying about something like that, and it's this unfulfilled promise that sparks Isabel's desire for freedom. But that's not all. Miss Finch also taught Isabel to read, even though teaching slaves to read was considered an "odd notion" (2.17).
In the end, both of these seeds Miss Finch plants take root in Isabel's escape from the New York. Her brief foretaste of freedom after Miss Finch's death makes her want it more than ever, and it's her ability to read and write that lets Isabel bring it to reality.
By reading Common Sense, she's able to understand the rationale for the wrongs of slavery, while filling out the false pass in Lockton's office is the act that not only brings her dream of liberty to pass, but allows her to be "reborn as Isabel Gardener" (44.101). Miss Finch may never have known it, but her wish for Isabel's freedom is ultimately fulfilled.
As for why she didn't just go ahead and free Isabel and Ruth while she was still living, well, that's anyone's guess, but it's probably safe to say it puts a damper on some of the good she helps do after she dies.