Property (including land) can be owned by two people together as either 'beneficial joint tenants' or as 'tenants in common'.
Where there is beneficial joint tenancy neither person owns a specific share of the land. It is their legal 'partnership' that owns the property and, upon the death of one person, the other person automatically acquires the title to the whole of the property. (It is completely irrelevant as to whether the deceased person had children or not, or as to the contents of any will. They can't bequeath what they, as an individual, never owned in the first place. The property has to go to the surviving person in the partnership).
If the two sisters were tenants in common then each of them would own a share of the land (which would, of course, be a half of it unless specified otherwise). Upon their death the terms of their will would determine who their share of the land would go to. If they died intestate the intestacy rules would apply.
This verifies my information about the two types of ownership:
http://www.landregistry.gov.uk/public/guides/public-guide-18
To see who inherits under the intestacy rules, refer to this flow chart:
http://www.makeawill.co.uk/clients/c4ca4238a0b923820dcc509a6f75849b/laws-of-intestacy.jpg
Inheritance Tax is paid upon the estate as a whole, not upon any specific part of it, and is paid directly from the estate before distribution of the residue rather than by any individual. It only applies when the value of the estate is over £325,000 and doesn't apply to property which is passed automatically to the surviving partner of a beneficial joint tenancy. Start here for further information about Inheritance Tax:
http://www.hmrc.gov.uk/inheritancetax/intro/basics.htm