Board Thread:The Flash discussions/@comment-26618197-20191120114526

''Barry turns his attention to prepping Elongated Man for life after Crisis and without The Flash, but Ralph ultimately teaches Barry a lesson. Meanwhile, Cecile faces her own moment of self-discovery while helping a recuperated Chester P. Runk reclaim his identity.''

So yeah, Nash had a daughter who died that Allegra reminds him of. You can tell by his interactions with her. 