I dunno about you, but single conversations with a sibling can DEFINITELY completely change their relationship. Heck, I wrote off my own sister because of one a couple years back, and only interact with her when necessary.
Single moments between adults, when taken seriously, can and should have meaningful impacts on the involved parties. There've been several single-conversations in this comic that have done that, even with characters that had long relationships. Granted, there aren't many characters that have 20+ year long relationships with each other in the cast...