My dad died 3 years ago from cancer, I was 24 at the time. Let me first say that it sucks, and I feel for you, but you don't need to be told that!
I was lucky in a sense, because my mother-in-law (I got married a week after he passed away; it was a busy time in my life!) also lost her dad early in life, and she helped me figure out what I needed to do. Obviously this situation is a little different, but bear with me.
The main thing you lose when you lose a parent is a person to ask questions, and a person to tell things. The telling you need to do yourself - if you're not a family that finds saying "I love you" easy (like mine was), give yourself a run up to this as it takes time but needs doing! There might be things your sister has always wanted to say to your mother - embarrassing things, personal things, confessions (of guilt or love!) - she needs to say as many of them as she can, because anything you don't say will prey on your mind forever. Trust me on that.
The asking if something you (and probably more your elder sister) can do on her behalf. There are things that we ask our parents as we grow up - what was I like as a child, what were YOU like as a teenager, what did you like to do when you were my age, why are boys (or girls, I'm not making heteronormative assumptions!) so goddamn frustrating sometimes... a lot of these questions aren't things an 11-year old child is going to think about, so making a "Mom-FAQ" of sorts for when she grows up will be useful. That answer you linked to makes reference to video diaries etc. etc. Memories are really important. I would talk to your mother about it as well, ask her whether there's anything she would like to store for her when she's older.
I know these are tough things to ask, and there's likely to be a lot of crying involved at least at first (if there hasn't already), because doing stuff like this forces you to face up to the limited time you have left, but I guess in a way that's good unto itself because it stops you being in denial. I know I was for a very long time.
I know most of this relates more to the "after she's gone" help than preparing her for the death itself, but if I'm honest there's not a huge amount you can do. The death of someone close to you is a world-shattering experience. You get through it, eventually, but it fundamentally changes the way you look at the world. I cry every time I watch a film where someone loses a parent, no matter how briefly the characters have existed. There are adverts on TV that I have to leave the room for now because they cut too close to the bone. I miss my dad so much, and I get through it by remembering all the good times I had with him, but nothing really could have prepared me for how it's changed my life.
Just... be there for her. Try and get the answers she might want, talk to her about her mother, don't let it become the elephant in the room. You will all be stressed, you will all get tense, you will all be dealing with more grief than anyone should ever have to, and you are going to snap at each other, and be mean, and take things out on each other unfairly. It's just the way it is. You fill a room with upset people and it's hard not to have a fight every now and then. She may not understand what's going on to the same level as the rest of you, but she's going to be affected by that in different ways - she won't have the luxury of knowing why. It's going to be up to the rest of you try try and keep the ship as steady as possible - don't rely on your father or your elder sister to do it, it's going to need everyone old enough to understand working together. Apologise when you need to, don't let your emotions bottle up, help your little sister talk about things so that she doesn't do it either.
That's it, really. There's not much more I can say. Everyone is different, so you could find that none of this really applies to you, just do your best not to lose relationships with anyone else in the process, that's the important thing. The best way you can help your sister is by making sure she'll always have a family. Hopefully something in there is useful!
