I'm ignoring the "Strahd's sister" bit. While the Izek thing seems contrived, the idea of Ireena having a creepy stalker other than Strahd seems appropriate for the tone.
I honestly cannot figure out why they would have put that in there. Admittedly I haven't read through all of the campaign yet, but so far I can't see how it adds anything to the adventure or even how it would impact it at all. It seems like a weird thing to put in there when it doesn't have a payoff - almost just a twist for the sake of having a twist. Except that this is a twist that would pay off only for the GMs in a lot of cases because I don't see how it comes up in play in a way that the PCs figure it out.
Having read the whole thing, I can say that I don't see how it adds anything at all, really. More to the point, I don't really see at what point the PCs would ever learn that information, or why it would be shared with them.
Actually, thinking it through, I can think of one reason why they put in the Strahd's sister bit: because they just wanted to have a reason -
any reason - for Madame Eva to 1) be in Barovia, 2) help adventurers, 3) have been doing both for a while.
In the original module, she's just there as a sort of set piece with nothing else explained. In 3.5, she's explained as being one of the three creatures with ties to the land. But in this one - especially one that tries to "explain" SO VERY MUCH about HOW Strahd got powers and WHY he's doing what he's doing and all the other needless explanations (like Ireena's actual parents), they would want to explain why Madame Eva is in town as well. And if you think about it... why the hell
is she in town? And helping random people try to stop Strahd? And not dead because of it?
Now, I think the sister reason is contrived, but having
something sort of helps, even just with motivation and conversation. But I actually think the Ireena one is worse: there's even less need in my mind to come up why Ireena is there, and to me the natural assumption that she's not really from Barovia was more interesting. (Also, it's a pretty non-story - "she's an orphan from the next town over whose brother is evil." You'd think they could have made it better.) Then again, I do remember my players lingering on this question for at least a minute last time we played Ravenloft...
Anyone have any good ideas for substitutions for either? The 3.5 version seemed a bit better: Eva was one of three hags who guarded the three fanes which were ties to the land. She was guarding the "forest" one, and I want to say that she and her sisters were the original holy guardians of the fanes but all three eventually got corrupted into hags due to Strahd's presence in the land (maybe I made that up?), and while her sisters were evil and mad, respectively, she was ok and was trying to get Strahd out. Different take on motivations but ultimately similar enough. But without the fanes this makes less sense.
Ireena was always just there somehow. I think I did some convoluted time-travel thing or something weird last time I ran it (Tatyana fell to her death and was reincarnated by a druid into a baby and then transported forward in time by fairies and anyway it seemed cooler at the time). Other ideas that might fit? (You can always keep the Izek plotline regardless of the family tie.)