"Hey, Dipper?" asked Mabel. "You know how I'm older than you by about five minutes?"
"Yeah?' asked Dipper.
"That means I'm technically your big sister, right?" said Mabel.
"I guess so." said Dipper with a slight sigh. His sister's very slight height and age advantage had been something she had teased him about quite a few times before.
"Well..." said Mabel. She looked down and let out a sad sigh, taking Dipper aback somewhat.
"I've just been thinking that there were a lot of times when... bad stuff happened to you, like last summer with the... puppets, or the... apocalypse. And probably some other stuff too. And I didn't help you, at least not at first." said Mabel, being vague as possible in an effort to not bring up unpleasant memories.
Mabel's eyes welled up as she said "And, you know, that's not really a good thing for a big sister to do. So..."
Mabel put a hand on her brother's shoulder and said "I just want you to know, that from now on, whenever you face something scary or bad or whatever, I promise you won't have to go through it alone."
With a smile, Mabel gave Dipper a big hug and said "Because I'm your big sister, and I'll be there to protect you."
Dipper was confused at first, but after a few seconds happily returned the hug, saying "Thanks, sis."
