Okay, Sanji stomps his brothers. That accomplishes what exactly? Is Sanji attacking the people working with Big Mom going to make her just forget about the bad blood between her and Luffy? Or maybe that will convince her to let them all go, not get angrier, and not attack them? I mean, we see that she absolutely despises Lola, her own daughter, for costing her the deal of a lifetime that she REALLY wanted, to the point that she's wishing that Lola, again her own daughter, was dead. But yes, I'm sure that Sanji, a complete stranger who is the subordinate of a man who's already pissed off Big Mom by taking her favorite thing(Sweets), ruining the wedding by beating his brothers down would make Big Mom just SUPER willing to let bygones be bygones, right?
Or maybe the assassins the Germa have at their disposal will hear about Sanji beating his brothers, and then ignore their contract to kill Zeff out of the goodness of their hearts. Or perhaps they'll respect Sanji and go work for him instead? Hell they'll probably help the Strawhats take down the Yonko, because let's not forget about Kaido, who must be just as ecstatic as Big Mom that the Strawhat's took out his biggest business and made war with him. Because after all, if the strongest Strawhat barely beat that Yonko's subordinate in Dressrosa with the help of dozens of people and a Supernova of the Worst Generation, then it makes sense for Sanji to assume that Luffy can handle the man who's super stronger than him with a super bigger crew at his disposal, right?