Dependent Parent Housing Benefit

My mum lives in a self contained annexe on the side of our property. She is 66 and claims guaranteed pension credit.

I have been told she cannot claim housing benefit. How can this be when she would receive it if she rents an annexe in the property next door. Is this right ?

Is she likely to be entitled to claim any other benefits. She has no income or savings.

Alternatively am I entitled to claim any tax relief for having a dependent relative living with me ?

Grateful for any help