Your mother-in-law's estate will have had IHT applied to it based on the rules at the time she died, and on the way her estate was disposed of under the terms of her will.
When the step father dies his estate will be subject to IHT on the rules applying when he dies. However, from what you say he does not own the house outright. Presumably he only has a life interest in it under the trust so the capital value will not be part of his estate.
The rules applying to IHT and trusts are complex and depend on the type of trust among other things. You need to get specialist advice if you want definite information.