ID3 - can brother-in-law be verifier?

ID3 form guidance states verifier "must not be related in any way" to person they are verifying. Is my brother-in-law (my sister's husband) classes as "related"? He is obviously not my blood relative but related by marriage, not sure if that means we are legally related... (he is a chartered accountant and I'm keen to use him!)

Anna - he would be classed as being related so falls within the definition 'in any way'

Thank you Adam. 

I'm assuming I can use my brother's girlfriend (who he lives with and is the mother of his children, they are not married), who is a GP?