Ouch! I wish I could tell you that you are a horrible person and this never happened to me but I cant. Even important things, REALLY important things that I care about deeply get lost in my head. To stop it from happening in the future you might want to get a white board (you know, the ones with erasable markers) and hang it someone that you pass multiple times in a day. I keep mine between the kitchen and dining area. Anything really important that I MUST remember goes on the board. Without it I am mush.
As to the family, the only thing that I can suggest is telling the truth and try making it up now. Explain about your brain and jump in with all four legs. Take them food, go over and be in charge of where to put things that people are bringing, answer phone calls, do whatever it takes to show that this was important it is just that your illness caused an error. They may not understand but you can make it up