Tenant's relative moves into the flat

Possession - Section 8 and Section 21

I found out from the tenant in the flat below mine,  that my tenant is no longer living in my flat. It transpires that my tenant has allowed her cousin and friend to move into the flat, maybe until March 24. My tenant did not inform me of this change and probably didn't intend too. I have spoken to my tenant who has explained that they would like to keep the flat and move back in March after visiting their unwell parent in Eastern Europe. Any thoughts / guidance about what i should do, would be most welcome. The tenancy agreement expires in Sept 2024.