Not if she leaves a will leaving everything to Save The Children Fund, like my uncle did.
If she leaves a will, then her estate will go to whoever it's left to. If she has no will then you could make a claim and her estate will be put intestate.
I presume then relatives would be searched for, but often it can take months even years before you would receive anything.
I don't want my sister to get a penny of my estate, so would rather leave it to charity.
As Velvetee says, if your sister dies without making a will, then you as her only living relative can apply to the Probate Office for Letters of Administration, to settle her affairs, i.e outstanding bills etc. Any residue would eventually be allocated to you. It is far better if you suggest she does make a will, it saves a lot of bother.