guests of tenants in HMO house

Other

Hi, I have an HMO house shared with 6 people, 2 of them are living in a double room as a couple. All bills are included. Recently, Tenant A told me that tenant B has got a guest living in her room and it's been about 1 month already. I asked the tenant B, and she said that that was her half sister who was helping her going through a tough time, and her sister is not living in, just being with her a lot during the day time and will going home at night.
I have no evidence to show that tenant B has a guest living with her apart from the utility bill went up dramatically since tenant B moved in. Tenant A claimed that she saw they cooked together, but it don't mean anything.

The HMO license only allows 6 people living in the house.

My question is if I should do anything about it? If there WERE someone living in tenant B's room, does it violate any rules?

Thanks in advance for any replies.

J