My confusion with Madam Eva's motives comes from the following passage in the Curse of Strahd adventure, when describing the Vistani of the Tser Pool Encampment, which Madam Eva is one of (p. 36):
Although the Vistani in this camp are in league with Strahd, they attack only if the characters provoke them with threats or insults.
So, the Vistani of this encampment are in league with Strahd, but on the other hand, on p. 37, as part of the same section on the Tser Pool Encampment, it describes how Madam Eva helps you understand how to defeat Strahd via the Tarokka reading.
The Tarokka reading is a major plot point, as I understand it, since the party cannot know where to go/look out for or how to defeat Strahd otherwise. The Tarokka reading itself is described in detail on pages 11-18.
In other words, the Tarokka reading is a major part of the story, and Madam Eva is the means to deliver this information about how to defeat Strahd to the party… and yet she's in league with him.
How does that make any sense?
As a footnote, this is not a designer-reasons question because I do not require designer commentary to answer this question (although if there is anything from a designer out there that would provide some insight into Madam Eva's motivation and willingness to help the party, I won't turn it away either; the main point I'm making here is that I just want to understand what is written, not to specifically seek out more from the designers).
Best Answer
Madam Eva's motives are complicated
First, regarding your notion that Madam Eva is in league in Strahd as she is a Vistani in the Tser Pool, note the description of Madam Eva from the appendix D:
Further, the true focus of Madam Eva seems to be to:
Because of this, her actions and motives can seem to be a bit contradictory, note this description also in the same appendix (emphasis mine):
So she does want to help him, but the way she wants to do so does entail helping the adventurers and you can't exactly say that she's in league with him in the same way the Vistani are.