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Don't forget trusts which may have been triggered on the death of a spouse.
When my FIL died, a trust was triggered to share the house with my MIL and the executors.
When my MIL died, we found out that my BIL (a literal world class legal academic*) had fucked the probate up, and no set up the trust.
You might well imagine the CGT and IHT shenanigans that this prompted.
We had a local solicitor (a literal market town class legal professional) sort it out.
* and yet, still a massive fucking idiot.
Do you know who owned the property i.e. who is named on the deeds as it may not be as straightforward as you think.
Your late grandad may have added your mum and her siblings onto the deeds at some time in the past and should have undertaken a valuation at that time. Now that he has died and the property is in your mum and siblings name there may be CGT to pay on the increase in valuation from whenever the deed of ownership changed to today.
This will also be based on any percentage that your mum owes e.g. it may have been grandfather owned 50% and the remainder split equally amongst the children.