Joint tenants

My client is a joint tenant of a 3 bed private rented property with her brother. Client has a dependent child who also lives with them. For their portion my client would entitled to the 2 bed rate of LHA and her brother the one bed rate (he is over 35 years old).

She has an agreement with her brother to pay 60% of the rent as she uses two of the three bedrooms.  I am aware that normally for Universal Credit where there are two joint tenants they would apportion the rent 50/50 but wondered if it was possible to apportion it differently due to different use of accommodation due to household composition.

Yes. See para 24(5), sch 4 UC Regs and the example at para F3197 of the ADM which is almost exactly the same as your case:

Warren and Jason are brothers and are joint tenants of a three bedroom property with a monthly rent of £725.00. Warren also has his daughter living with him. The DM divides the rent between the number of tenants on the agreement given them both £362.50 core rent. Warren asks for this to be looked at again as he actually pays two thirds of the rent. The DM considers this to be a reasonable arrangement and revises the original decision to make Warren’s core rent £483.34.

Brilliant, thank you.