Two big things for me:
COMPOSITIONOverall, my biggest issue with the comic has to do with the layouts and composition, or more accurately the general lack thereof. I get it that the comic started out not as a comic but illustrated text, but that does not explain the new pages having no layout and really it was never an excuse for the earlier ones either. This tendency in the comic to show every panel as a wide shot without ever bothering to find more interesting compositions, is a very “Poser” thing to do. It's also a very lazy thing to do, and it does detract from the impact of the artwork in many ways. For example, compare this death scene from
Dark Sisters to this one from fellow Drunk Duck comic
Curse of the Black Terror:

See what I mean? Composition is everything. It's not enough to just show that things are happening; a good artist can actually put the reader there and let them
experience it. There has not been a single moment in
Dark Sisters that I have felt like I'm experiencing the events rather than voyeuristically watching things happen. It's always felt more like a diagram of a comic than an actual comic, and that's entirely because of the overuse of wide shots and the lack of page layouts.
And I know from that, um, previous incident (
*cough*) that sometimes people take it the wrong way when they receive criticism. We;re not “attacking” the comic; we're pointing out things that can be made better. We're trying to help.
CHARACTERSThe characters are not very engaging. I understand Caitlin and Roxy as character types, but they never feel fleshed out as actual people. It's like I know who they are and what events happen to them, but I never feel like I know them the way I know for example Juno MacGuff or Bruce Wayne. I'll never find myself saying “that is such a Caitlin thing to say,” or anything like that. There are a few moments when the characters do seem to briefly come to life, but for the most part they don't seem to think, talk, or act like real people. They're character types, there to serve a purpose in the story, and that seems to be pretty much it. That may well be adequate for this type of story, and your readers don't seem to mind, but overall I think that both this and the layout issues contribute a certain amount of narrative noise that creates a barrier between the reader and the story, preventing them from being as immersed in it as they could be.
When you have a comic where the artwork is basically a diagram, and the characters don't have any real presence about them, it's sort of like the difference between reading the summary of a novel and reading the actual novel. Sure it's adequate to get the point across, but there's a lot missing that could enrich the experience.
SUGGESTIONSDark Sisters has a lot going for it. It's a girl-heavy cheesecake comic, which is always popular. It's a fairly straightforward mystical adventure, not too offensive or challenging to the reader insofar as subject matter goes. Anyone should be able to pick this comic up and be able to get into it; it has universally enjoyable qualities… well, for anyone who isn't a prude, at least! By and large, the audiences here at DD and elsewhere on the net aren't all that demanding, especially when they're getting your comic for free and it has lots of nekkid women in it. But as much as they may be happy with the comic as it is, they'd be a lot happier if it were done with a bit more finesse. Two quick recommendations:
#1: You need to step outside your comfort zone and try experimenting with image compositon, and maybe even try putting some pages together as actual comic layouts. It's hard, I know, but it will pay off. Look at some other comics here on DD that do a good job of visual storytelling and try to learn from the things they do well (Curse of the Black Terror is an excellent example).
#2: I know this sounds counterintuitive at first, but when it comes to characterization, sometimes less can be more. Stop relying on thought balloons to get across what your characters are thinking. This is a classic example of telling when you should be showing. There is never a good excuse for using thought balloons in a comic, and if you find yourself feeling the need to rely on them, then that means you're failing somewhere. What your characters are thinking should be clear from their expressions, posture, actions, and the things they say.
Stop relying on blocks of narrative text. If you're doing a good job illustrating the story, your readers should be able to figure out what's going on without you verbally explaining it to them. And as much as possible, avoid using too much expositional dialogue. Again, you should be showing and ot telling wherever you can. It may seem counterintuitive, but the way you present your comic, showing every last detail in each panel and bombarding the reader with narrative text, thought balloons, and explanatory dialogue, actually ends up keeping the reader more distant than if you'd simply allow the story to tell itself naturally unencumbered by all the narrative noise. Also, I believe that relying too much on these narrative devices is a crutch that can actually prevent you from developing both as a storyteller and as an artist.
These are, of course, only suggestions, and your comic appears to be doing well enough as is, so don't feel any need to rush out and start changing things just because a few webcomic review people said you should. Ultimately, you should make the comic that feels right for you.