In my opinion Spilight is sweet, because I see Spike and Twilight as friends and that's it. Not once has Twilight ever refereed to Spike as brother or son. Seriously he is not her son for two reasons; one he is a dragon and she's a pony, two she was just a filly when she hatched him, in other words she was between the ages of 4 and 10, in human terms. Obviously the age gap doesn't seem too big, so I just don't see how that could work. And, yeah yeah yeah, I know she hatched him, but that doesn't make her his mom. Whoever laid the egg is his mom. And let me emphasize on this, SHE WAS A FILLY! That's like saying Applebloom could be a parent! THAT'S SICK!!! So obviously the "son" concept has a few holes in it.
The next idea that bugs me is how everyone thinks they're brother and sister. No, because yet again there is a species barrier.
Plus he is her assistant and best friend, nothing more nothing less. He was given to Twilight when she was a filly to be her ASSISTANT, not adopted by her. She always says "Number one assistant" not brother or son. And she says "he was my first real FRIEND" not brother.
So in my view the whole relationship between Spike and Twilight is a friendship. That's it. They are not related in anyway, via son or sibling. Not just because of the species barrier, but because they have never once refereed to either of them as being anything other than friend or assistant.
Also there is one more thing I'd like to point out. [link]
You know because you've all totally looked at your sister or mom like that before.
On another note I do NOT support Sprarity simply because I find her rather stuck up and that she doesn't really seem to care about how Spike feels. Honestly I'm a little glad that they laid back on that shipping in the show...









Everyone has the right to love someone as long as she/he love that one back!
Of course she's not his mother. She herself was just a filly. A filly raising a baby just wouldn't make sense. I don't know what the writers are planning to do about spike's backstory because Lauren said it was Celestia who did the raising (though she gave spike to twilight, because she was the one who hatched him, and that the two of them were meant to be a duo). Though Lauren said that this was simply a concept that was never officially added into the show yet, so the writers may change it. So I dunno what the writers will do...
I do in fact think they play off as goofy little brother, and strict older sister. And here's my reason: Yeah twilight doesn't say he's her little brother, and yeah she calls him her assistant. But Dragon Quest pretty much confirms this to be true. Spike says, you're more than friends your my family, then the ponies all hug him showing that they feel the same way:[link]
Even in Cathy Weseluck's interview she says that he sees them all like sisters (except Rarity of course) I don't know what part she says it, but the interview's here:[link] (scroll down, you'll see a play button, but it takes a while before they get to the interview though)
As for them not calling each other "brother-sister" I have a theory on that too:
I think spike just doesn't want to say it because it's sappy (you know him, trying to be Mr.manly man
If you like spilight, that's okay. It's just not my cup of tea. I just wanted to add in my two-cents.
But I won't spoil your fun. If you like it, more power to you.