Would attending my niece's wedding, approximately 20 days after my father-in-law's passing, be considered disrespectful to my wife and her family?

There is no legal offense, according to Islamic law, to your wife's family in your attendance, or your wife's and her family's attendance, at your niece's wedding. A woman's mourning period for anyone other than her husband is only three days, and what is prescribed for grief and weeping over the deceased should not exceed three days.