Jaycee, you say your SIL had a lease for the first year but now does not have a lease. What did the original lease say would happen after the first year?
It may be that Mr B originally had the lease from Mr A for the entire area but, for whatever reason, has now decided to assign part of the property to your SIL. In these instances the original tenant (Mr B) will still be held responsible for the rent being paid to Mr A which may be why the rent is being paid to Mr B.
I suggest you get your SIL to establish all the facts he can and consult a Solicitor who is familiar with the Landlord and Tenant act 1954.