Most unlikely to go to back into the pot of 18 (and if it does, it will be a coincidence rather than the application of a rule).
Their shares form part of their estate(s) and fall to be distributed according to whatever rules apply to their estates;
So, if they left wills, those wills will determine who benefits.
If they left no wills, the rules on intestacy will determine (likely their siblings would take their estate, with the children of any precedeased siblings taking their late parent's share - but it needs to be carefully checked)