Another IHT question

blobbyrob

Would appreciate some help please if possible in layman's terms.
Sadly my sister recently passed away. Her husband passed away almost 5 years ago. They had mirror wills and everything was automatically passed down to my sister when her husband died including their home which they owned mortgage free. My question is regarding what the IHT allowance would be in this situation? 
My understanding is it would be £325k but then I read it could be doubled up. This where I'm confused.

Quote from moneyadviceservice.org.uk.
Couples can transfer any unused NRB when the first person died to the survivor.

This can double the amount of NRB available up to £650.000. This extra transferable element is known as Transferable Nil Rate Band.

Hope this makes sense. Cheers.

Keep_pedalling

Her husbands nil rate band will be transferable in full so yes it is doubled up. If they had children then her estate has the potential to have an exemption of up to £1M using the residence NRB.

blobbyrob

Many thanks for clarifying Keep_pedalling.