Hi Thanks for your question, I am Sam and I am one of the UK tax expert here on Just Answer.
This is not a benefit - a benefit is when an employer provides additional money or monetary value - like free accommodation, company car etc along with employment
But this is family, and you are not their employee or are you?
I am shocked that a business handling tax disclosure has not idea about employee benefits and when it does not constitute this. Especially if
1) They are handling your tax disclosure and
2) Asking you to find out information for them
Perhaps you are best seeking support with a company more expert in their field?