>>7669455>>7669431>>7669531also, I imagine after 10 years of fighting her you and your family might be worn out. many parents, especially mothers, can fall for narcissist's tricks and give them pity because of guilt; but this is a mistake that enables them. it's important to remember that your family is not to blame for this, does not deserve this and shouldn't take any of this. if they say that "it's your sister after all", tell them she doesn't care about the damage she does to the family. she likely doesn't have any compassion for you and doesn't care what she put you through, denying any fault whatsoever. she's a grown up and needs to take responsibility. you can't ever satisfy her neediness even if you tried. if you wish happiness for her, you deserve to be happy too. your family doesn't exist for your sister alone.
I suggest you and your family read some books on narcissism. if nothing else it will make you feel like you're not alone and get confidence and strength (you're gonna need it).
lower contact with her if possible. reduce and control any communication with her. keep her in check constantly. keep talk to business matters. don't let her derail it and make it about herself.
be warned that she might do things behind your back.
last, it's tempting to believe your sister can change. but it's extremely hard to change a narcissist, and nigh impossible if they don't want to. very often this lingering hope to bring back the person close to you turns into denial. and this will provide them the last straw that they won't hesitate to use. don't fall for this and remember what you're doing is for the best of your sister and of your family.
if your sister is beyond salvation, it is extremely tough but she needs to be isolated. no contact is the only option. it is tough and grieving will have to take place. but it's for the better of your family and all the people damaged by it over the years. time to focus on yourself and heal the damage.