Move over, Meghan: There might be another American in the family! Ann Ukrainetz, who lives in Escondido, Calif., claims she found old love letters that indicate she’s the product of her English actress mother’s affair with Princess Diana’s grandfather, Maurice Burke Roche. Ann, 75, reached out to her royal grandnephews, Princes William and Harry, in 2008, but got a letter back saying they “regret they are unable to become personally involved.” Now, she says, the family has left her “no option” but to have his twin brother’s body exhumed to prove her blood relation! “It’s almost like a theft of birthright,” says Ann. “I do believe if Princess Diana were alive today, and she knew this, she’d be knocking on my door.” And we thought Meghan’s dad, Thomas, was their…