Why Incomparable's sister would not be Impregnable

If Britain built a second Incomparable-class battlecruiser, would HMS Impregnable be a likely name?

No. Contemporary connotations and conservative Royal Navy naming practice make Impregnable unlikely; Indomitable would be more plausible.

The Royal Navy of the period was conservative about ship names. Fisher might secure Incomparable, but Impregnable carried connotations that newspapers could exploit whenever the ship and its crew deployed. A shore establishment could retain the older name with less public attention than an active capital ship. Indomitable is a more culturally plausible alternative for a sister ship.