The sliding scale that Nether Edge describes kicks in after 3 years and is actually a reduction in the tax payable rather than a reduction in the value of the gift - OK comes to roughly the same thing.
And (the big gotcha) is that the sliding scale ONLY comes into effect if all gifts made 3-7 years prior to death total more than the IHT threshold at the time of death.
http://www.hmrc.gov.uk/cto/customerguide/page1 3-1.htm
The other potential trap is that if she continues to live in the house she must pay you and your brother a commercial rent. Otherwise it's a gift with reservation and the Revenue will regard her as still being (at least part) owner.
As I said first time round there are loads of complications - I'm sure not all mentioned - so it's essential to seek expert advice