>did not think it was relevant to even ask me, or she knew it and actively avoided my opinion. Both situations are alarming.

You're right to be concerned, but maybe a little enthusiastic about influencing her decisions. I would gently urge her with what facts I could but maybe go easy with the big brothering. Sometimes letting people make their own mistakes beats the risk of chasing them away with overbearing guidance.

Of course I don't know anything about your relationship with your sister so YMMV.