You cannot use your husband's CGT allowance as he is not an owner. Every individual can only use his own allowance.
When you sell the property, you will calculate the Capital gains value by
Minus: Purchase price + legal fees+ selling costs+ construction costs
Answer: Capital gain value
Dividend by 3 people, as your mother is no more.
Your share will be further reduced by the £12,300 CG allowance
The Net amount will be taxed under the CGT rates 18% or 28%