You could warn gram you might be late, go to the party if it is at 3, stay for 1-2 hours and then go to the family gathering-this is the extroverted option and I know you are an introvert, but if the conflicted feeling is too much it is an option. You will be tired at the end of the night but you could just say you won't be over on Sunday and take Sunday to do absolutely nothing at all and stay home to recharge.