It depends on your view like Authrix said. Besides desperate times call for desperate measures. I'd gladly sacrifice the sisters in exchange to save billions of imperial lives.
No, the stupidity of it is that Grey Knights, the most purest untainted warriors in the Imperium, needed to shower in the blood of Sisters of Battle, who are known to have fallen to Chaos on many occasions, because the Grey Knights weren't pure enough...
Yeah, I'm just gonna point out that, according to the Sister's codex, none of them have fallen either. There's one sister in a Bl novel. Still doesn't excuse bloodtide though.