Caring for grandmother

[Hanee987]

Hello
I will be applying for my husbands visa soon, however I have come across a small 'problem':
I am currently in receipt of carers allowance through caring for my grandmother. An immigration advisor has advised that I will need letters from my aunts stating why they cannot care for their mother as it not a strong case on my behalf (or something along those lines!).
How true is this? Will I really need letters from other "potential carers"? If this is the case and I do not include these statements with my husbands visa application will it be a rejection?