Is Harry related to Voldemort?

[Sylvee Estel]

My friend and I came up with this theory...
Harry could easily be related to Voldemort. I personally think Voldemort could be Harry's grandfather. James Potter's dad. Voldemort should be about 57 years old in book 5. So the age works. This could also explain why Voldemort wanted to kill Harry in the first place. He could have been afraid that Harry would inherit his powers, and eventually overthrow him, or something like that.
The only problem with my theory is that Harry should have the same last name as Voldemort, which would be Riddle. So maybe James Potter was adopted, and his last name was changed? Or maybe Voldemort is related to Harry in a different way. Maybe he's his uncle, or something like that?
Or maybe, Voldemort is Lily Potter's father. They makes sense too, becuase all the rumors say we will find out something big about Lily in future books. Also, rumors say that Harry's and Lily's green eyes are important. Slytherin's house color is green. Voldemort is an heir to Slytherin...? The only problem with Voldemort being Lily's father is Petunia. Wouldn't she know if her father was a dark lord?
Anyway I just thought this was an interesting idea... any thoughts?
I know I have way too much time on my hands...