She Drives Me Nuts But I Love Her Like A Sister!!

Joeys sister drives me so nuts!  I love her as if she was my own sister, but she is SO unresponsible and flakey!  She constantly asks me to do things, and I don't mind doing things for others, but then she backs out.  And you think I would learn, but she comes through just enough that I keep doing it.
Example.  Marlow got a new bed, so I was sending her old one to the dump, and she asked if she could have the mattress.  Fine.  I set it aside in a room, where it blocked off the bed AND the dresser that held my clothes because she said she would grab it the next day.  And then she wouldn't respond to my messages and calls to come get it.  I've been shifting it as much as possible to get to my clothes and I NEED that bed for my dayhome.  So finally today, after a week and as half, she messages me and says I can get rid of it since she has a toothache and her head hurts.  Oh yea, because now I need to find someone with a truck AND pay money for the dump we DO NOT have because she wanted it soooo bad and then backed out.
 
I'm supremely pissed off and at a loss with what to do with it.  But she is like a sister to me, so I'm biting my tongue and giving her just enough cold shoulder that she gets my point...and she will.  But she doesn't have her ducks in the same room let alone in a row and she lets things that are not life or death slide most often.  It's so annoying!!