@Background Pony #0D7B  
My thing is, Twilight pre FiM was in no way fit to be a mom and everyone around her, ie. her parents and Celestia would have known this. There is no way they would have left the primary caregiving for a newborn to a socially stunted kid who had just been accepted into the most academically rigorous school in her city. I’ve seen some of those kids in gifted schools. They eat, sleep and breathe academics. Straddling child rearing on top of that would be plain bad. That would deprive both Spike and filly Twilight of a healthy childhood. What is more common is for older siblings to have a hand in raising their younger brothers and sisters.

@Background Pony #0D7B
 
It’s also not considered a mother/son relationship in any culture on Earth.
 
It’s normal for siblings, when the parents are absent or dead, for the elder to take on a great deal of caretaking for the younger, and (in media) this is especially commonly seen with older sister / younger brother.
 
But that is still a sibling relationship.

@Background Pony #0D7B
 
To put it simply: she’s probably been his guardian, he looks up to her, but she was too young to raise him when he was a baby, period.
 
Might be a bit similar to Lilo and Nani, with Spike and Twilight, minus dead parents. They’re generally rather complementary in their dynamics (for instance, filling in for her so she could take a vacation), so it leans more on “siblings” anyway, aside from it being outright stated.

It’s both. Spike is basically Twlight’s adopted brother, however she also raised him in part. This is hardly complicated or unusual.

Spike is Twilight’s adopted brother, the way it should be.

@Background Pony #40A5
 
It was automatic to me to just assume they had a sibling relationship.
 
However, I had assumed that they were boarding with Celestia (due to attending her fancy university) and that she would be Spike’s primary mother figure, as she was for Twilight before Velvet and what’s-his-face got more screen time. It seems that Twilight’s parents play this role in the modern canon.

I don’t like overthinking things so I’ll just stick to brother and sister.

I don’t do complicated.

Twilight never (onscreen) vocally expressed herself as Spike’s parent - he’s the one who implied that by bringing up “my real parents” with only Twilight present, totally unprovoked. This would imply that Spike recognized Twilight as a parental figure.
 
This, of course, is harshly contradicted by Sparkle’s Seven where Spike, Twilight, and Shining all readily and explicitly recognize it as a sibling relationship.
 
It’s probably just inconsistency caused by different writers.

It’s easier to just say he’s the #1 Assistant.