Hi Fransesca
It don't think you have much evidence that aspergillus could be causing this yet - it is one of the rarer reasons why dogs develop runny noses - see [
www.aspergillus.org.uk]
Go for treatment for the more common infections - the fact that a litter mate may have a similar problem could mean that they both have nasal passages that are slightly more vulnerable to infection - it could be a structural 'defect' they have in common rather than an immune defect