So, they brought up how Big Mom can (and has) killed the relatives of people who piss her off. First was that one guy who was invited to a tea party and rejected it for his sick dad. Big mom beheaded the dad and gave it to him in a present box. Then we have Big Mom threatening to kill Zeff if Sanji doesn't accept the marriage.
So, what exactly is stopping the other yonko like Blackbeard (or Kaido prior to his fall) or the WG from going after other friends and relatives? Kaya? Nojiko? Makino? Franky's gang? Hell, why didn't Big Mom kill Zeff as revenge on Sanji? They're more than capable of doing these things, and they have no moral qualms in attacking innocent people. The literal fucking devil runs the world government, I'm sure they can at least arrest their friends/family for "helping a criminal."