The main approaches I have taken for characters are:
1. Pre-planned relationships
I want two characters to have a particular relationship and so they just do. The reason for this can be something as simple as them being siblings gives one or both characters traits I want or I think would be interesting. The same kind of approach works for wanting two characters to be dating or married or just friends. People have relationships of different types and sometimes I want/need a character with particular kinds of relationships.
Deciding which way a relationship goes relies on a number of factors including:
- What you want out of the relationship for the characters
- What you want out of the relationship for the wider story
- What the relationship would mean for the story
And for all of those reasons, whether that impact on the story is what you want. This is a decision that has to be up to you though you could make a pro & con list for each type of relationship and decide that way.
2. "Discovered" relationships
This comes down more to how I write a story in that I can set a scene and environment and have vague points I want to head towards but then it feels like the characters start to get more involved in how things play out (and how their relationships are/which ones they have).
In this case I might be writing a scene where the character goes home to get a book from their room but then during that interaction with their parents their sibling stops by and says hello, asking about the major events of the story. Well, I might not have pre-planned the sibling but they (and their sibling) kind-of made their relationship exist (in a way that feels natural but also out of my conscious control).
This kind of situation can even come up with respect to two pre-existing characters knowing each-other. I had a story where there was a nanny character and an admiral who had not interacted "on screen" but I realized overtime that the admiral was the nanny's grandfather and that the nanny had applied to be the nanny to learn from someone their grandfather deeply respected (without their name and real identity muddying the relationship from the beginning). That was a family relationship that seriously impacted those two characters but felt natural for who they were and helped explain motivations and interactions that I had already written and that I was yet to write.