How do you explain the fact Ichiji is 186cm, Niji is 185cm, Sanji is 180cm and Yonji is 194cm? A slight difference in height in identical twins/triplets/quadruplets is possible, but such a disparity in height, with Yonji being significantly taller than all his siblings, doesn't match with the idea they're supposed to be identical.
What scientific studies showed was that identical twins are more likely to share the gene responsible for alcohol/drug addiction because it's already encoded in their genes. So of course, if the twins are nearly identical genetically, they'll likely share that gene. They just inherit it from a parent. It's very different from how they're affected by external factors, which mostly comes down to chance.
In Sanji's case, it was made clear the drug wasn't powerful enough to be completely successful and affect all of them, so it only worked on Sanji.