My 16 year old sister-in-law is wanting to commit suicide. What can I do/ say?

Sorry in advance, I will probably ramble and not think about proper grammar. TL;DR my sister in law told her close friends that she has cancer, so they will think that's how she died, instead of suicide.

So for a 16 y/o she's been through a lot. Her parents aren't together, mom got remarried to a man who ended up being a coke/steroid/sleeping pill/ alcohol addict loser. Before we all found put he promised Mom in Law he would be the bread winner while she had his child. He drained all of moms savings, put a lot of the family in quite a bit of debt. In and out of rehab stealing money, bought a fancy brand new truck. He convinced mom to sell their current house ( moms wanted to rent it out) while they gutted and planned to redo an older house. No they live in a not even half finished house (no doors, half done bathrooms no baseboards/ molding around windows). Also, sister in law quit school (doing online classes) b/c of a girl bullying her. This means sister looks after baby sister while mom works 12 hr days for minimum wage. All while trying to be a 16 y/o girl. Which isn't easy for the most privileged of girls. I know a lot of people get through this, but it's just background for the story. So obviously sister in law is confused and no wonder she is depressed.

Most of my family had dealt with depression/ mental illness so I am not a stranger to all of this, but I've never had to deal with a suicidal person. I guess last night she went around telling her close friends that she had cancer, and didn't have long to live, so that when she killed herself she wouldn't have the "shame" of suicide. I've known that she has thought about it, as many people do, but I never thought she would seriously think about it. This is a HUGE red flag to all of us and for the time being, she isn't to be left alone. Now I'm a younger sister in my own family, so the whole "big sister" thing is new to me. I try to give her the best advice I can but what do I say? What can I do? I feel so helpless. Her brother and I moved in to help them pay bills so I am always around her. We are very close but I feel as if she has put herself in a plexiglass bubble, she is pushing us further away and that worries me even more.
Sorry if this is the wrong SubReddit, and if it is please tell me where would be a better place to post!