To be fair though, we know by now that Kawaru's transformation is a huge problem. Moreso that her partner could trigger her (and others) with his blood - this is even applied to strong willed beasts who already went through harsh training.
It's easy for us readers to just dunk on Utsui, but if you have a family member under constant threat of permanently turning into a beast, endangering themselves and others around them, you'd desperately want to try to stop that possibility, sometimes by any mean necessary.
Now, I wouldn't say her methods are okay, but there are little she could do at this point. Rather than waiting for some evil reporters to suddenly expose her sister one day and they wouldn't have time to prepare for the press, the cops, the scientists, the government spies... to ambush them, she might've thought sabotaging at least give them some sort of control (and hopefully convince her sister to just give up). At least that's probably her mindset, which is morally wrong but still somewhat understandable).