I always considered them as bother and sister. Simply because of the way they treat each other.
Anyway, how old could be Twilight when hatched his egg? Form 4 to 7, if we guess she was about to attend the magic kindergarten,(or primary school at least), but If we consider Twilight as a gosh darn prodigy then it could be 2-3, but she was significantly bigger than that. I say she must be 4-5 when hatch the egg.
She could be any kind of prodigy, Celestia would never be so stupid to leave a baby to her responsibility at that age.
I guess she was "compulsory" adopted by the Sparkle family by Royal order. It can explain why the "family" is not treat him like a true family member.
But my head cannon is that Celestia took care of him. Therefore he and Twilight has such a good relationship, because Twilight spent significantly more time in the school than home. I imagine it like in the Harry Potter, she was lived in the magic school and went home might be three times a year, so she spent most of his time with Spike and Celestia,
Of course, Celestia involved her in taking care of Spike, because it was good for her having company around - Twilight was inclined to be antisocial - and helping caring for an other living creature taught her responsibility, what she would need when became a princess.
It explains why Twilight loves so much Celestia; technically she became her mother for years. ( In my headcanon, She was basically taken away from her family to prepare her for her destiny)
Sure, she loves his parents she remained in touch with them, but it was not too intense. Shinning Armor is a different case because as a royal guard, he had more chance to visit his little sister.
That is why, Spike and Twilight has such an intense bonding while Spike have no with the Sparkle family.
Damn, I wrote a lot of bosh, here. XD
Anyway
The people voted and decided.