My mother's in law funeral

Ok I need advice am I wrong or what should I do. OK my mother's mother in law just passed away. Mind you I've only met this woman 3 times and both were for an hour or so at family gatherings involving my step dad's family. I have Saturday off (only day in 8 I'm off) we have family plans to take my son to a sports picnic. Well my mom is angry and wants me to drop my plans and drive 2 hrs to go to her mlawa funeral. (Because it would make her husband happy if I go) ok here's the back story.... when I got married my mom's husband refused to let my mom pay for anything or even come around to help me with the wedding. (She snuck and paid 100 towards my cake) so why should I drop my family plans on my only day off to go to her husbands mothers funeral or pissed them off and stay and spend the day with my family?