Your IVA should not impact adversely on family members who have no financial attachment to you. However, in reality there may be some discrimation by the bank or it just may be the general conditions in the lending market.
If there is an issue your family should demand to know why and in the meantime get copies of their credit files to show anyone who has a problem.
It sohould definately not have anything to do with your credit file, and if it does your family can file a notice of disassociation with you which should do the trick.
