Okay so since I think that oldest/youngest/middle in the Hargreeves family is pretty amorphous and they all pretty much ooze into whatever gap is left empty in the moment...
I am thinking about Klaus and Allison giving Lila the Shovel Talk.
Because they're the most experienced with relationships, right? Like ultimately they've both had good and bad relationships and they know how to pick out a bad person from a lineup and they love Diego FIERCELY and also know he is an open hearted dipshit so they are going to, with all their charisma and edge and people skills, scare the living daylights out of Lila so she knows if she has to end it with Diego she BEST NOT BE DOING IT CRUELLY
Except. Somewhere in the conversation it comes out that Diego is the first friend that Lila has ever had, and Klaus and Allison are simultaneously in big sibling mode AND best friend mode, right? So they can't have that. Absolutely not.
What I'm saying is that a conversation that starts as a Shovel Talk ends as a sleepover with nail painting and hair doing and finding out they have similar personalities and probably drinking and roller skating, ill advisedly, and by the morning they absolutely adore her and they're giving DIEGO a Shovel Talk, too.