UC housing non dependant or joint tenancy

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Hi, could someone help to understand this, please?

My mother receiving PIP - daily living component and mobility both higher rates and ESA. My sister always lived with our mum as mum needs care almost 24/7 specially at nights, sister not an "oficial carer" doesn't receive carer's allowance. Mum rents privately two bedroom and receives housing benefit but this time sister's name is on mum's tenancy agreement, whereas she's never been named in contracts before. Sister does't pay rent and the agreement does not states that she needs to nor it states that sister is a joint tenant. Mum received migration to UC letter and they're not sure- would this be a non- dependant or joint tenants ?

Thank you very much for your knowledge and help.