i'm sorry to hear this... unfortunately(though not everyone), once they have been using excessively with drugs such as amphetamines, or really anything that is truly mind-altering, a good majority will never snap out of it.
my mom has been a crack addict for many years. she does nothing but mooch off my grandmother who is nearly on her death bed, and yet it does not phase my mom, she just continues to throw a pity party, her own problems as her mom's fault, and blame everyone else for her mistakes. she will use people(i have heard some very awful and really... sickening stories about her), that i'm sure have screwed up a lot of young women by now. she believes that the government is after her, that aliens very well exist on earth, and that she has not totally lost it, as a matter of fact, to her she is perfectly normal.
it is very sad. everyone will cast her away, treat her like dirt, and yet she doesn't even realize it anymore. if someone called her something awful, she probably wouldn't remember it or... just overall not take notice. she wouldn't take notice to the staring eyes, weird looks, and people whispering and giggling about her. it is very sad... and it hurts me so badly, but she will never know anymore, she will never go back to normal, i can NOT FIX her and it kills me, it is painful to no end seeing a family member so far gone... that even with help they will never be normal again, never quit lying, being the victim, and never realize their abnormalities, mistakes, or faults anymore.
all of this however is a stepstool for her, whether she realizes it or not. you see, my grandmother believes it is her fault why my mom is this way when in fact it is not. my grandmother has supported her through thick and thin without question, paid her bills at the age of 45, still continues to do so, and will not throw her out in fear of my mom living on the streets. my grandmother though, she is getting older. once she is gone where will my mom be then? i try not to think about it, but it's always a thought, it is always there.
she has put me through more than she will ever know, it is draining and hurtful despite how much i love her, but she is toxic and will never change. me personally, i had to quit letting her around as she believes i killed people, that i've committed crimes and am with the government, that me and my dad went on a killing spree together as well. i couldn't keep letting her around, it is just too much for me and my easily altered emotions. i wish her the best in life, i wish her well, i forgive her, but i can not have her with me.
i am truly sorry to hear this about you with your sister, your nephew, it is a very numbing and hurtful thing to see. i hope your situation is fixable and wish you the best of luck.