I feel bad, I accidentally started an argument between my girlfriend's parents.
How did you start an argument between them?
I made up a lie that they were going to cut their daughter out of their lives. No one appreciated what I said.
You shouldn't tell a lie around other people. Apologize and tell them you made a mistake.
I'm sure everyone will calm down and it will blow over.
You have to be careful what you say because you don't want to make up false information. This could upset people and cause arguments which could cause lasting rifts in the family. Try to smooth things over with everyone.