HLA typing between siblings to identify a genetic disorder
No, you can't conclude that. 

First of all, two siblings being HLA identical tells you very little about the rest of their genome. HLA genes are closely linked, and each haplotype is transmitted in mendelian fashion as a single gene.  For non-linked genes (on other chromosomes, or further away on chromosome 6), they are as or nearly as likely to have the same genes as non-HLA identical siblings.

Further, the disease of interest may (is highly likely to) be multigenic and have incomplete penetrance. The heritability of common disorders generally varies from 20-80% (Cecil Medicine Chapter 41, the Inherited Basis of Common disorders).