the whole cover story was about yonji and niji getting tortured by big moms underlings... the two silhouettes resemble raid suits. we know that Father, Reiju and Ichiji escaped WCI... so its probably the latter two saving their captured siblings...
besides why would blackbeard take the chance and invade a territory known to be impenetrable when we know that katakuri + cracker + oven + others are likely to be there? he would face considerable losses with questionable payoff