> Is "this interested" mean "interested to this extent"?
Yes, that's right. As Michael says, the meaning is similar to "so."
I would say that the difference between "so" and "this" is that "so" implies something excessive or more than expected, whereas "this" is a little more neutral, and simply refers to the specific degree/level/extent of something. "It's so cold" implies that it's very cold (it's cold to such an extreme/surprising degree), but "it's this cold" just implies that it is cold to a certain extent (it's cold to "this" degree).
I agree with Michael that "so" probably works a little better in your sentence, since it sounds like you're saying that she's unusually/surprisingly interested in poisonous snakes. But the word "this" is also fine, if the speaker and the listener both understand what "this" (degree of interest) refers to, in context.