I mean- attitudes about things can change a lot over time too. Weed isn’t generally viewed as seriously as it once was. Advice and expert opinions on parenting can change as can attitudes- so depending on when you and your sibling are born- by the time they do the same thing the attitudes about it and the tools for dealing with it can change. A kid who was a teen in the 90’s was still in prime ass whoopin territory, but a little sibling born in the early 90’s May have grown up in a “no spanking” house. Parents are people too and they also learn as they go.
So what was a shock and a huge disappointment with the older sibling- through experience and talking and reading they may have learned is just an average part of growing up. It’s even more true where you turned out well or not in their eyes. If they liked how you turned out- they saw that those things didn’t impact your future as much as they though they would. If they are unhappy with how things went they may think that whatever they did with you didn’t work how they wanted so they should try something else.
Being a parent the first time can be terrifying. Many people are in their teens or early/mid twenties when they have kids. Anyone in those age groups now? Have life all figured out and everything under control? Imagine not just having to keep this tiny fragile looking thing alive despite books and looks telling you the slightest thing from a certain food to sleeping with it or holding it wrong could kill it- but you’re also responsible and will be held responsible by it for how the rest of its life turns out- and everywhere you look you see people saying that the tiniest choices can make or break your entire future. Oh- and you probably love it so much you’d die for it. You love it probably more than your spouse/partner or the other parent.
Then you go through the whole thing and you learn that many of the things you obsessed over and tried to eat perfect? They didn’t really matter at all. Perhaps you see some places where you think you could have done better. You’re probably older, more confident, better established in life, and certainly more experienced. You’ve got the confidence that every other kid you raised didn’t get killed or become a war criminal- so some of that blind fear may we’ll be gone. Unless something horrible happened to one of your kids and then it may be harder being a younger sibling. But talking “averages” here.
I had the exact opposite: my sisters were kind of free growing up, and did stupid things. So life was A LOT stricter for me to prevent me from doing the same things. Which I would not have done anyways.
But it helps shape you for the better if you let it
TLDR: People, even parents, change over time and with experience.