(Mostly a next gen AU, but you could probably make it work with an OC in (C)'s place)
(Based off of Encanto)
In a place where ultimates (which were treated like superpowers) were a lot more rare, (C) is born, or adopted into the largest family in town..without an ultimate, (B), having married into the family and not having an ultimate..kind of led to them blaming themselves, of course.
The two ways the scenario I have can go is
(C) and (D) get into a big fight, which led to their lack of ultimate being brought up and (C) storming off. (B) witnesses the fight and gets hit with a wave of guilt that (A) has to get them out of when they find them off on their own (could also lead to both of them talking with (C))
Or
(B)'s in the kid's seat this time, has a fight with (D) this time and runs away from home, which in turn leads to them meeting somebody their age in town. (A) and (B) get along pretty well, which leads to more secret meetings throughout the years.