They're Fae, not fairies. They're kinda evil in this one story because, long story short, they had a kid of their own but he was born dead so they kidnapped you from your family...

Your mother and father just had you. They were over the moon, however... Cosmo and Wanda silently approached the cribs, they were looking at you. You were perfect and you were going to be their daughter/son.

They were your parents neighbour but in reality they were faeries and were manipulative, waiting for the perfect time to kidnap you.