Ahhh lovely another thread full of myths and misinformation on mental illness and mental health treatment 
Which won't be deleted because those of us that are mentally ill are fair game.
"Shes lazy and very selfish cault her out for lieing few times but never pulled her."
What are you basing this on? Do you understand anything about mental illness? Do you understand that it can be just as energy sapping/exhausting as physical conditions? That "laziness" can be manifestations of lack of motivation, fear/anxiety, avoidance patterning, plain not seeing the point in doing a thing?
Mental illness also often manifests as a focus on oneself. Tunnel vision, being unable or fearful of engaging with the outside world, anxieties making you focus on what you can manage, inability to empathise or sympathise or being fearful of doing so.
Lying can also be a factor in several illnesses/conditions, or it may be to avoid awkward questions or conversations or because you're feeling threatened...
So no you and your partner aren't best placed to help her because he's in denial about what's needed and you're hostile, ill informed on the subject and don't have sympathy for her.
Where is he getting the idea that medication is a bad thing? Does he think the same if he has an infection and is prescribed antibiotics? Doesn't he take painkillers if he has a headache?
She'd be better off either at the other relatives or back in hospital by the sounds of things.
She's clearly very unwell and needs intensive treatment, which may well include medication, therapy, practical support...
Your partner loves his sister, sometimes loving someone means doing something they don't like for their own good.
I think signing her out of hospital was a mistake and a knee jerk reaction. And one that needs to be reversed if possible.
I feel so incredibly sad for your sil. She must be so scared, and so desperate and so sad. She must feel as if she is such a burden, unwanted and unvalued.
How would you feel in her position op? How hurt would you be to be at the very lowest you've ever been in your life, so low that you think your only option is suicide, to genuinely believe that those you love would be better off without you alive?
What would you want your parents or siblings to do then? Who would you turn to?
I'm not shocked at this thread because I've seen several similar ones on mn. I'm saddened and disappointed and I think mners and mn can do so much better on this.
Most mners are intelligent, articulate, thoughtful people who are very supportive of those sick and disabled in other ways. But on mental health they are seriously lacking in empathy and understanding.