First you need to work out if your estate is worth more than the IHT threshold - currently £325,000 for an individual. If not then there is no IHT to pay. If it is higher then the IHT then tax will be deducted on the excess. A family member living at your house, therefore, would not affect the value of the estate or the liability for IHT.
There are ways to gift cash or assets, exempt from IT, to adult children before you die and they are explained here
There is a situation where - in an effort to avoid IHT - parents give their property to adult children (or sell at a very low price) but continue to live in the house, rent-free until they die. In that case, the tax due on the benefit i.e. free accommodation, would be deducted from the deceased's estate before distribution.
Rent-a-room relief, I think I'm right in saying, does not apply to family, only paying lodgers.
You may benefit from either seeing a solicitor or talking to CAB to understand how best to approach inheritance planning.