Provided those siblings have not previously received gifts/inheritances under Group B, then there won't be any CAT.
Sibling 1 will have a CGT liability if they originally paid less than 50k for those shares (subject to indexation, if they were acquired before 2003).
When siblings 2 and 3 dispose of the shares, they will be subject to CGT (currently at 25%) if the shares are worth more than value at the date of the gift (25k) plus the stamp duty paid (1%).