Is there any evidence which shows that Peter's mother knew Ego was an alien?
On her deathbed, Meredith tells Peter, "You're so like your daddy. You even look like him. And he was an angel. Composed out of pure light."
Meredith's nickname for Peter is "My little Star-Lord," which Peter later takes as his code name.
When Ego shows her his space seedling thing behind the Dairy Queen, Ego says that soon it will be all across the universe. Meredith says "I can't believe I fell in love with a space man."
And, most convincingly,
- When Peter meets Ego he says "My mother told everyone my father was from the stars."