With this alliteration, the author Edith Wharton is telling the reader something about Ethan's character. On the surface he appears to be "grave and inarticulate", especially in the isolated circumstances of his living and marital relations. Ethan was at one time a student of engineering, but was forced by family situations to assume life on a small farm. He is married to Zenobia, but she is bitter and a hypochondriac, and provides little positive interaction for him.
Despite all this, in the alliteration
"There was in him a slumbering spark of sociability",
the author is telling the reader that deep within Ethan's nature there is the inclination to seek "sociability", the give-and-take of true interaction with others. It is true that this inclination is almost undetectable; due to a long period of suppression, it is not vibrant, but is "slumbering". Like a fire that has been long deprived of oxygen, it is almost completely snuffed out - only a "spark" remains. Nonetheless, it is there, dormant, only waiting for a breath of fresh air to bring it to life again, a breath that is provided in the character of Mattie.
By using the sybilant sound of "s" repeatedly in this alliteration, the writer conveys the sense of quiet dormancy. The "slumbering spark of sociability" lies hidden beneath the surface, silent, but definitely present.