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TL:DR both sets of children have to pay a proportionate share of the IHT after the death of the second spouse.
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First off, I will say that there is an organisation called the Society of Trust and Estate Practitioners (STEP) which solicitors can apply to join. These people are generally very knowledgeable in this area. If you search "STEP solicitor [name of your town]" then that should provide you with a list of local STEP registered solicitors.
I would strongly urge you to approach a STEP registered solicitor.
(It may be that one or other of the solicitors you have approached is STEP registered?)
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Just so that I understand this correctly. You have previously lived in your house but you now live in your husband's house.
Your husband's will leaves everything in trust to his children (your step-children) but gives you a lifetime interest in his property.
If your husband dies first then his house passes to the trust and you are the life tenant. His IHT allowances are also passed on to you (unless he separately gives gifts to other people in his will, in that case you will need to deduct those eg he leaves money directly to his children on his death).
At this point, HMRC will now consider that the trust fund (your husband's house) is now part of your estate for any IHT purposes.
As long as you are leaving your residence to a direct descendant (eg children or step-children) then you can make use of the RNRB (residential nil rate band).
Assuming that your husband did not make any gifts on his death then you have the full amount of his allowances (ÂŁ500k) to add to your allowances (ÂŁ500k) so that you have a total allowance of ÂŁ1 million of which ÂŁ350k is the RNRB.
If you have two residences then your executors can choose which property to nominate to use the RNRB. So, as long as one of the houses has a net value of at least ÂŁ350k then you're fine.
Since both houses are counted as part of your estate that is going to be ÂŁ1.4 million (ignoring any other assets you've got). If you have your husband's allowances of ÂŁ1 million then that means that there is ÂŁ400k subject to IHT so that would mean paying ÂŁ160k.
This liability is then apportioned between what you leave in your own name in your will, your "free estate" and what is left in trust from your husband's will.
So that would mean that your stepchildren would have to pay about ÂŁ114k IHT from the ÂŁ1 million and your children would have to pay about ÂŁ46k IHT from the ÂŁ400k