[Caleb (OP)]

   I've wondered about this for quite a few years -- if my brothers or sisters get a flu, am I more likely to get it because of genetic similarity?
   Also, in a family dynamic (with the flu going back and forth within several relatives), will the flu virus tend to automatically change itself to the point that I am more likely to get it, as compared to strangers?
   If so, then probably local groups, towns, etc., would then also be more susceptible to catching it.
   Yours,
   Caleb

[CliffordK]

It's a tough question.

If you think of any epidemic virus, there will be some people that will be immune, and some that will be susceptible to the virus.  And these differences may be hereditary.

This may mean that some individuals, or families might be more susceptible to certain flu strains than others.

It is also possible that your family will have been exposed to similar viruses in the past, so the immune system might react similarly. 

Selection?  Survival of the fittest? 
Presumably the infection is from a relatively small group of highly related viruses expelled by the previous host. 
So, what is the mutation rate of the influenza virus?  This puts it on the order of 10^-5 mutations per nucleotide per infection cycle.  I think that was per infection, and not per replication.  Nonetheless, with about 1.5 x 10⁴ nucleotides, mutations and genome variability within an individual infected person don't seem to be a major don't seem to be a major adaptation of the viruses. 

So, there may be some selection of susceptibility to the initial virus infection, both based on your family genetics, and previous family exposures to the flu virus.  Beyond that it shouldn't be mutating to better infect your family.

And, of course, if your family is living together, then they would be in close proximity to one another, and thus have the opportunity to share the same viruses.

Fortunately I live far enough away that i don't have to share all the viruses my brother's school kids bring home!!!

[Caleb (OP)]

Clifford -- What a great answer! Sure seems you know your stuff.

One of the things I have considered for years is the issue of the virus entering a family member, and then being made more virulent to others of the same genetic background -- I think you address this clearly, indicating with mutation rate info (great stuff! -- I guess my fifth grade math teacher is right -- numbers are important! ) that what I am suggesting is probably extremely unlikely.

Thanks!

Caleb