Is it a sin to legally pursue the husband's family after they assaulted one with beating and cursing, and is the husband obligated to divorce his wife based on his family's request to reclaim his possessions?

A husband is not obligated to obey his parents in divorcing his wife unless there is a legal justification for it. Nor do they have the right to make his possessions a price for that, as his money is his exclusive right. The hadith "You and your wealth belong to your father" has specific conditions. There is no harm in pursuing legal action against them for their transgression, although forgiveness is more virtuous given that they are in-laws.