SDP and residing non-dependants

I have a case which is a Shared Lives placement (previous foster care). 3 people in the dwelling:

1. Shared Lives landlady/carer.
2. Shared Lives client - age 18 claiming ESA and DLA mrc/lrm. He has a licence for B&B, liable to pay landlady/carer and receives HB. No arguments about whether the arrangement is commercial or not.
3. Landlady/carer’s 30 year old son. He works and receives no qualifying benefits. He is not (and can’t be) liable to pay for his accommodation.

They all share use of the kitchen and living room.

Does number 3 prevent SDP entitlement for number 2 because they are not ‘separately liable to make payments in respect of his occupation of the dwelling to the landlord’?

I’ve a feeling I’m missing something important here but can’t think what (just back from leave is my excuse).

Thanks.