Tenant has sublet without permission

[fresh to the market]

I'm hoping somebody can help me.

I found out today that the boyfriend and girlfriend I let my property to split up within the first month and that she moved out and he immediately moved his brother in and has been sharing the house with him.

This information came from the brother contacting me, not either of the original tenants. His brother (my tenant) had injured himself and only paid half the rent this month as he was going through a claim for housing benefit because he can't work until he is fit enough. The brother called today to say the second half will be with me soon as his brothers HB claim had been paid and he could now pay his half. In theory of course I could demand the rest of the payment from the ex-girlfriend if I needed to as the agreement was also with her.

In the tenancy agreement there is a clause stating that the tenants are not allowed to sublet any part of the propery so I am covered there if I want to kick up a stink.

My question is, given that it is within the first 6 months, what are the implications? Should I keep the agreement as it stands or look to have a new agreement drawn up for just the original tenant and turn a blind eye to his brother being in there? Or get the brother credit checked too and then draw up a new agreement for the both of them? Or just keep the agreement the same, turn a blind eye to the brother being there on the basis that I'm still getting the rent and then change the agreement after the initial 6 month period is up?

Also what happens about the deposit? Neither one of the original tenants have informed me about the changes but as I now know I want to make sure I am going to be doing the right thing