Their relationship seems so complicated..mother just doesn't sound right but it does seem like Twilight acts like one towards Spike sometimes.

Twilight's like a big sister to Spike...though i also think of her as a mom since she was there when he was still an egg and all

She did seem to care for him since he was born. I can see why Twilight would be Spikes mother

I always thought Spike and Twilight were to friendly with eachother to be mother and son. Plus, Twilight may have hatched Spike, but she didn't exactly completely raise him. But they have grown up together, and Twilight is a bit more protective of Spike than a best friend, so I'd say sister.

I can definitely see some kind of brother/sister style relationship between them.

I also think they have a best friend relationship in addition to this, but that's less personal as I also believe that all seven of the main characters have this relationship with each other.

Mother, because she raises him and takes care of him. Also, she bosses him around, so definitely a mother XD

It's obvious that Twilight is Spike's mother. Why? Who hatched the dragon from the egg? Twilight did. Who took the dragon into her life? Twilight did. Who raised the little purple dragon from the day he was born up to this day? Twilight did. You think she is his sister?! Why do you say she is his sister?! Those who say that are dead frickin' wrong! If your going to say Twilight is related to Spike, then the correct answer should be that she is his mother! Just proving a point.

No wonder Twilight treats Spike like a son.