To step up to the plate means to take action in response to an opportunity or crisis, and in this context she is taking action by taking responsibility for her granddaughter.
Context: She might take action and responsibility once she knew what a crappy job her son was doing of raising her granddaughter.
Hypothesis: She might step up to the plate once she knew what a crappy job her son was doing of raising her granddaughter.

Given this context, is the hypothesis true (Yes/No/I don't know)?