I felt like talking to them about bitcoin and how's of guaranteed security than any of those schemes or cryptocurrency. But when I looked at their countenance about this socalled scheme I was moved to hold my peace and let them learn from the hard way...
You failed them, I would say. You failed them in your capacity as a big brother and as someone who knew better. You should've warned them first to clear your right to do good and your conscience. At least, trying to talk them out of it and telling them about Bitcoin would remove any sense of guilt from your mind in case they eventually get scammed (that's usually the end result of such investment). Everyone doesn't have to learn the hard way, especially if they had those to educate them around.