I have 3 DDs, age 10, 5 and almost 2. Their father and I separated 2 years ago and he subsequently disappeared from our lives after having been pretty detached from us all for some time.
Recently, my eldest keeps undermining my parenting. I think she thinks I'm too hard on her sisters but actually, she is causing an awful lot of upset.
Some examples are:
I was playing a chasing game where I was a bad guy and DD(5) bit me hard enough to draw blood. I told her off, said I wasn't playing the game anymore and that she could no longer ride her scooter home as she had been desperate to do. A minute later I turn around and DD(10) is hugging and consoling her, and telling her she can ride her scooter home!
DD(5) asked to use the iPad and I said no, with a valid explanation. She ignored me and climbed up to get it anyway. I asked her to return it and she refused. I pointed out that she couldn't use it anyway as she doesn't know the password and that I wouldn't be letting her use it at any point that day if she refused to listen. She refused to listen so I removed it and told her she couldn't use it until tomorrow at the earliest. 5 mins later, DD(10) is hugging and consoling her and putting her kindle on for her.
We were in a public toilet and I was washing my hands. DD(5) was trying to open the door but it was too heavy. I asked her to wait while I wash my hands but she continued to try to open the door. I pointed out that if someone tries to open it from the other side, it will hit her. She rudely shouted that it wouldn't, I said that it would and asked her to please come away from the door until I was ready. She refused and someone did indeed crash the door straight into her head. I consoled her but I did point out that if she had listened, it wouldn't have happened. DD(10) was carrying her around and fussing her for ages, saying the lady was mean for bumping her head 
DD(2) is obsessed with pens. I tell her that she can only have them if she's sitting at the table with some paper. DD(10) keeps passing them to her whenever she shows an interest, resulting in the sofa having been drawn on.
The inconsistency is really confusing for the younger ones. If I tell DD(10) that it's my job to parent and to not challenge my decisions, she huffs off and says she was just trying to help. I point out that if DD(2) draws on her stuff, or if DD(5) breaks her kindle because she tries to use it without permission then she'll be upset so to leave me to sort them out but she keeps on doing the same sort of things.
Tonight DD(5) had found my phone and was asking DD(10) for the password and DD(10) was pretending she didn't know. She knows full well what it is and that DD(5) shouldn't have my phone but seems terrified of saying no to them. AIBU to make her stop undermining me, or am I indeed being ungrateful for her help?