I'm going to lecture my younger sibling about his spending habits
Would your parents lecture him in the same way?
They are more experienced and they know how to budget better so probably yeah.
Your sibling will learn on his own. It's not your place to teach him about finances; your parents can do that.
I'm just going to lecture him anyways because it is fine with me.
You can do what you want, but he might resent you for taking over a parent's role. Be a sibling to him and not a parent. Help him instead of lecturing.