Is the relationship between a wife and her husband's family considered an act of upholding the ties of kinship (silat ar-rahim)?

Affinal relations (in-laws) do not become blood relatives (arham) to the wife unless they were already her blood relatives to begin with.

May Allah reward you for maintaining ties with your husband's relatives despite their ill-treatment, and beware of giving Shaytan an opportunity to sow discord among hearts. For affinal relations constitute a mighty bond. Indeed, Allah the Almighty has coupled it with the bond of lineage in His saying: "And it is He who created from water a human being and made him [a man of] relationship by lineage and marriage; and ever is your Lord competent [in creation]."