How can I ever make up for this?

I feel bad.  Not the kind a person gets when they ate too much or feel a cold coming on.   No, that is the best thing that could happen to me right now.   it was one of those "could have done's but life somehow got in the way" events I live for.  Everyone gets them but it is too late to retract the situation.  Here was my little dilemma:

The day before I always plan out what I am going to do.  I do this by looking at my calendar, place in my mind any events that I need to attend to, and then it gets done.  On occasion I do not put it on the calendar.  The reason is I do not have all the facts to formulate an entry on my calendar.  I could have gotten a reminder for one of them, and it was all my fault.  Wednesday nights I usually go to bible study.  Well I did not feel like going, because I was upset with Saturday.  Long story, not going to explain.  Anyway, I did remember that my pastor's grandson was having a book signing yesterday.  This was his first book signing, and I wanted to go to support him.   I also had a business meeting yesterday morning.   So I had thought to myself Tuesday evening that I will go to my meeting Thursday morning and the book signing Thursday evening.  My day was completed, I can come back and work on the mini book swaps I need to mail off next Friday.   I had lost the flyer, but no problem, I thought, because I would be reminded of the book signing at bible study, and I can get the location there.  
So yesterday, I did not feel like going to my meeting, so I had a "work in my PJ's " moment.   5pm hits, and my sister calls.  she tells me that mom was going to babysit her kids, keep them overnight, so she can go today to put in some applications.   One thing about my sis is that she calls you at the last minute, expects you to drop everything, and come to her rescue.  OK , that is not how it really happens, but that what it feels like to me.   So she tells me, "What time are you picking up the kids?"  She does not ask me if I WILL pick the kids up for mom, but "what time?"  I blatantly told her that I had not planned on picking up any kids,  just like I did not say anything to her Wednesday night about me babysitting today.  I am the type of person you have to ask me if I have the time to do things for you.  I do not mind doing things for people, if they ask on time, and ask me if I am doing anything first.  I am also the type of people who expects a person to not get mad if I have plans, and I had plans on both occasions.   This chick also does not have a working cell phone at the moment for me to call her back.  She gets mad at me, and tells me that she will call me back.  (she is using someone else's phone).  My sister never calls me back, I missed the book signing, and now I feel bad about the whole thing.  On top of that, I had to call mom, who with dad goes out to dinner (WHAT did you forget that you were babysitting?)  So they get home, call me, and I tell them what happened (minus me forgetting about the book signing), she ask me about the number my sister called from, them mom calls me to tell me that it was not a working number.  It was working a second ago.On top of all of that, I cannot find the paper stack I am supposed to use on one of the books, so now I have to take the time to go get another one.
Today better become one beautiful day and fast.

So how can I make up for this?  for starters, learn to say no more, then my sister will learn how to except it more. (Just because I am single without kids that does not mean I can jump went you ask me to do something.  I do have a business to run, as well as other things I would like to do).  Do not beat myself up just because I did not do something I planned to do. Set ground rules.  My family tends to forget that single people have things to do too.  Lately, expect the unexpected.  My sister is good at this one.
\Well good night.