Is there a mod that lets you get out of killing everyone in the 'purification' quest? i'm at this point in my game, and i don't want to lose the original family members

You could just kill them to get the journal updates (or just get the journal updates) and then just resurrect them all.

But as far as your question goes, I don't think there is.

No need for such work, there's http://www.tesnexus.com/downloads/file.php?id=6189, which lets you "whisper a warning" to your fellow DB members.
There was also a mod by CDM that was simply called "Skip Purification", which did exactly what it says on the tin. Not sure if it's still on Planet Elderscrolls, but that's where I saw it before.

Both of those look interesting. Which one would recommend ?

Whispered Warning is good stuff. Not just a "skip me" option, but involves at least some level of interaction, and you can be selective about who you spare - leaving that rude cat to rot in the Void for example.

Alright. Thanks!!

But the rude cat decides to become your friend just before you kill him

Yes, but that's proof of his duplicitous ways and justification to gut him where he stands. :ninja: