Wobbly legs
I'm afraid your sister will not be able to make a claim for a Contribution Based benefit as this will require Contributions in the two Tax Years 2016-17 and 2017-18 and she will not have a full year for the latter one.
Providing her circumstances allow it, she can make a claim for Universal Credit. If she has started her cancer treatment by then, then should have automatic qualification for the LCWRA group.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems