I grew up with fraternal triplets as cousins. There is one they consider the “baby”, and there’s one who’s widely treated as the oldest, even though they were born literally less than two full minutes apart (emergency C-section). I also had two friends who were identical twins, and another two who were fraternal twins. Neither set of twins had an “older” kid except as a joke when we were little. When they were 8, the girl of the set of fraternal twins developed T1 diabetes. After that, her brother started calling himself her “big bro” and taking on more of a caretaker role.
From my experience, it seems to depend a lot on whether the parents treat one kid as younger or otherwise baby one kid over the others, and also on whether one kid has a health issue. In both cases where a “baby” developed, the “baby” had a health issue (CP and spina bifida in one case, diabetes in the other), but also, in the fraternal twins, the kid with CP and spina bifida wasn’t the only kid with health issues, just the one who got babied for it. If the parents treat one kid as younger, the kids will pick up on it and respond accordingly (which seems to be what Walky is taking advantage of here), even if they don’t register it consciously.
Side note: In every family that plays favorites that I’ve encountered, the kid(s) who’s not the favorite always knows who the favorite is. The kid who is the favorite almost always thinks they’re treated fairly. If that aint a great metaphor for privilege dynamics in society, I don’t know what is.