Dramatic I know, but that is how I feel. I just don't know what to do.
She begged me to help her get a job because she is skint but her partner works shifts and she has four children, making childcare difficult. She figured if she got a job in my place then I could do her shifts when her partner is working and she'd do some others of mine instead. Which is fair enough. I was happy to help. She's my sister and I adore her and hate seeing her struggle.
She has now got a job in my place. Her shifts are Wednesdays (split shift) and Thursday day. I am off Thursday night anyway.
She never fecking well does any of her shifts and I am fed up. I feel like she is taking the piss.
The boss is off this week and she hasn't picked up enough to be able to do her shifts unsupervised, so I have to work this week for her. He's back next week, but she can't do her shifts then either, because she is going on a holiday a-fucking-gain. For the second time since getting her job, so I'll have to do those shifts for her too.
When she gets back her partner will be on nights for two weeks so I'll have to do those shifts too.
So for four weeks I will see my kids only on thursday evening and a sunday
while she swans about on holiday with all of hers, even though she's that fucking skint if I don't get her a job she can't afford to give her kids a birthday 
I want to ask for her to be sacked so that someone who wants to work can take her job and I get some time off.
AIBU? I feel awful, but I cannot keep working every god damn night so she can work when it suits her. Dd1 now cannot join family judo with me because it's on one of my sister's nights, which I always have to do. I have had to cancel the dog's training class which is something I needed to do to help my future job prospects (I am starting a uni course in dog training and wanted to get first hand experience of group training classes) because it is on one of my sister's nights. Her confidence or lack of means she is not picking the job up quick enough to any weekends because she'd be too slow to pick it up, so I can't have any weekends off.