Question

Why does Inspector interrogate Mrs Birling before Eric?​

Answer 1

there is not enough information for me to give an answer

Answer 2

In my opinion, this is because Eric was "with" Eva, and as Mrs Birling also had an interaction with Eva, and one that ended poorly for Eva, Eric may have felt remorse at her behaviour. Therefore, the Inspector did this to show Eric how cruel those with power are. This is my interpretation anyway.