I-751 Child Filing Separatedly Question...HELP!

[Mishumishu]

My sister needs to remove her green card condition, and she got a notice after she filed the I-751 that it was missing our step-father's signature. In the requirements for children filing separatedly it does not mention to submit the parent's signature, but we are wondering if perhaps it the form that was completed incorrectly the first time.

On part 4, where asks for info on "the parent through whom you gained cond residence" she had my mother, who is still a permanent resident. I think it should include my stepfather's name instead, who is the petitioner, no? Please let me know if this is correct.

We will also include a letter signed from him, but I'd appreciate any enlightment you can share on this. Thanks!