I'm just about caught up and haven't seen anything addressing it yet so I'm theorizing Zeldris and Estarossa aren't Meliodas' brothers in the conventional sense. I think they may have been formed from him in some way, i.e. those two are like the two halves of Meliodas given physical form.
There's a few reasons why I think this may be the case. Meliodas' wanted image looks like Esta, which makes me think maybe before Meliodas' "betrayal"(And before his personalities gaining form) that's how he looked hence the poster. Esta has a full counter that works on physical damage which is a mirror to Meliodas' magical full counter. We learned Meliodas actually had 2 commandments at one point, Piety and Charity, Esta is Charity and Zeldris is Piety. Zeldris has the same physical appearance as current Meliodas which may imply he is the "youngest" brother of the three(order being Meliodas, Estarossa, Zeldriz). Lastly we saw in purgatory Meliodas' father mentioning taking his emotions or something to that extent, if he's taking bits of Meliodas' being each time then it's not that far off that he'd be repurposing those parts of Meliodas.