Informant interviews - your experiences?

Hi all, 

I'm new here, so apologise if this has been asked countless times before. 

I'm currently going through my assessment via NHS and have reached the stage where the assessor has asked to speak with my family about my childhood experiences.

I know this is a usual part of the process, but I haven't told my mum and sister that I'm going through it. I'm reluctant to do so, mainly because I haven't had the best relationship with them (we're ok at the moment). I doubt mum even understands what autism is.

What exactly do assessors ask about? Is it based on what I've already told them? What have others' experiences with involving family of origin been like? 

I have been told that they don’t have to be involved if I'm uncomfortable with it, but I'm also concerned this might impact the outcome of the diagnosis.

Any input shared would be much appreciated.