And we have no idea when Therie was most recently in heat. So it's entirely possible that she's already pregnant (or will be within minutes due to what she's already done - let alone her plans for tonight).
It has long been known that humans and keidran are mutually infertile. Under even the most expansive plausible interpretation of what Neutral changed, there hasn't been time for much proof that things have changed - yes there are pregnancies contradicting it, and probably lots of them particularly among teenage humans and teenage-equivalent keidran, but nobody's seen the newborns yet because none have been born yet. That leaves miscarriages, and probably there hasn't been all that much study of what fetuses look like at various points.
Further, we've been told that the tigers live relatively isolated. When word does get around, it will likely be slow to get to them... except to the extent that it originates with them.
And I get the impression that Dahlia approves of (or at least doesn't strongly object to) her daughter's actions.