I think the benefit you are referring to is this one.
http://www.direct.gov...inganestate/DG_179372
But it doesn't apply in your father-in-law's case; it only applies when assessing potential IHT liability on the death of the second partner - when one can use the first partner's unused IH threshold.
In your father-in-law's situation he can give away some of his assets to whoever he wishes - no problem. However an IHT implication in respect of the gift arises in the event that he does not survive for 7 years following the gift. If he did not live for another 7 years, there would be an inclusion of part of the market value of the house at the date he gifted it in calculating the IHT liability for HIS ESTATE. There is never going to be any tax liability on your wife or her brother (however, stating what is probably blindingly obvious to you, if he left the residue of his estate to his new partner, a big chunk of it might be removed to pay the IHT liability arising from the gift of the house before the new partner was able to inherit the residue).
The seven year rule is called a 'potentially exempt transfer' by HMRC and there is some stuff you can read up about it here - including the reducing scale of liability as years go by from the date of the gift until the seven years are up (when the liability falls to zero).
Because of the complexity within the family of what he is thinking of doing, I do really think you might do well to advise your FiL to take tax advice before embarking on this - though I trust these links anbd explanation give you a decent starting point.
BM