Urgent help needed understanding HMO

HMO Licensing

I have a question regarding HMO. My AST is coming to an end this month and I am creating a new contract with a small rent increase.

I have let my house to a couple as main tenants. I have mentioned in agreement that I am allowing them share the house with his cousin and her husban as Permitted Residents(to be considered as tenants households) and maximum 4 people can live in the house.

Now his cousin and her husband are moving away due to work relocation and my tenant has got another cousin to share with him. All are have surname and coming from same area.

Initially I was going to do same contract and thus I agreed. Now on second thought, I am worried if it's forming HMO and I may face issues. 

I am not earning/operating as HMO but just letting tenant to share the cost of renting house with cousin. However, on the same time, I don't want to face coucil charges or tenant recalling rent of last year.

Agent in the area advised to just get a declaration from Tenant that he is cousin and will share the house.

I will appreciate your valuable input.