Family, Friendship, Socialism

As has been previously discussed, in the German Ideology, Marx-Engels attempt to distinguish between 'natural' and 'voluntary' relations, using the family as an example of the former.  Now, one curiosity in the discussion is that there is no explanation of how the husband-wife connection, which is the product of some contract, is 'natural'.  Instead, there is no such curiosity in a much clearer illustration of the natural-voluntary contrast: Family vs. Friendship, i. e. because the relation of siblings is biological and involuntary, as well as not necessarily amicable.  So, the use of "brothers" and "sisters" in reference to the members of a Socialist society is antithetical to the presumed voluntariness of the latter, whereas "comrade" is more appropriate.