Can my wife sign my brother’s son’s passport? ( Her nephew by marriage not ny blood) She has the same surname. If we weren’t married it wouldn’t be an issue.
Can his step grandad? He has different surname.
They both have the correct qualifications.

Not the way that I read it, they’re related by marriage…

No. The counter signatory must not be related by birth or marriage (including in laws and partners of family members)

Bit silly really but I guess them the rules.