Good vent, PH...
I know you wrote you feel pity for your sister, etc....but right NOW what I'm sensing is that you feel more frustrated and even angry...and puzzled.
From what you've shared there are definitely some deep-seated issues going on with your sister's self esteem over things that go way back to childhood. You're right people want/need validation and get that need met however. Right now it's the pothead boyfriend. Also, many folks get back at parents over long ago hurts by being passive-aggressive. I'm not knocking her for that. In lieu of being able to be direct, people can sometimes take the passive-aggressive route instead.
You're disappointed in her and can't get too excited or happy about some of her decisions lately. That's totally understandable, IMO.