Not at the moment. Only partners can share parental leave under the current arrangements, not grandparents. However, your mother could make a request for flexible working to change her working hours to be able to help you, as long as she has 26 weeks' service.
If she wants her hours to be changed just for the short-term, she needs to make this clear. She could also ask for dependants' leave if you find yourself unable to manage your childcare in an emergency.
When the shared parental leave regime was first introduced, there was talk of it being extended to allow working grandparents to share leave. Commentators are reporting that this proposal has now been shelved, at least until the completion of a government review of shared parental leave.