Inherited property in two stages / dates

My 2 siblings and I inherited half of the family home (ie around a 16.5% share each) when our Dad died around 20 years ago. He left it to us in his will. None of us have been living there and Mum has continued living at the family home on her own (no rent or mortgage) until her death last year, when we inherited the other 50% of the house and are now the 3 equal owners of the property. We are selling the house and our solicitor has recommended we check whether any CGT is due on the proportion of the house we owned (ie 16.5%) from when we originally inherited Dad's share of the house nearly 20 years ago, based on the increase in value of the property since we inherited. I'm struggling to locate a scenario exactly the same as this on the forum. If CGT is due, then how do we make this clear on the Govt online system ie that we are paying/calculating CGT based on the 16.5% share? Does the system allow for additional information/text to be added to explain? Can you advise please?

Hi,
You will give the date acquired as the date your father passed away. A value will have been applied at that point and you take your share of this. You then add on your share of the new value at the time your mother died and this is your purchase price.
Thank you.