How can I unlock my iPad?

I got an iPad mini 16GB from my sister-in-law who upgraded. I wanted to port over my iPad mini 16GB that's older into this new device. I took the following steps:

1) Erased contents and settings of new iPad (after using her passcodes to disable Find my iPad.

2) Securely backed up my current iPad.

3) Plugged the new iPad into iTunes and restored from backup rather than set up as new.

4) It gave me a white screen with a "Slide to upgrade" message. The 4 digit code it asked for wasn't my 4 digit code, nor was it my SIL's, nor was it 4 others I tried. Now I'm locked out of the iPad for 60 minutes. We really only have 2 codes for iPads, so I'm not sure what's up.

How can I access this iPad without trying 1 code every 2 hours? I don't even care if I have to set up as new at this point. The only thing I can think is the backup is for an old iOS (old iPad couldn't be updated lately) and the new one is up to date. But I can't figure out this unknown access code.

How can I just wipe this thing?

I still have the old iPad with all my data on it.