Some people are very solution oriented, they like to "fix" things and try to resolve problems in a simple, straightforward manner.
However, life is not simple and issues are typically not black and white, but exist in multiple shades of grey. Sometimes a person just wants to express their pain and open up to someone, as you're doing with your sister, without seeking some sort of resolution/advice from them or trying to be swayed in a particular way.
Help is a very vague word, and rarely is it clarified what does this nebulous concept of help even mean. When some people vent, they might be expressing frustration and vocalising their pain is a type of catharsis- simply being able to talk freely and feel understood can be a type of help to relieve one's burdens. Posting here is a good example of this.
When others do it, they might be looking for advice or suggestions for resources, ways of solving a problem, etc, and may explicitly ask for someone's opinions. I think many people feel pressured to try and "fix" suicidal people they know, even when it's not possible and may even make the situation worse if they say certain things.
The discussions around suicide are so controlled and usually make it difficult to leave room for nuance, and many people have been conditioned to believe that if someone is suicidal for a long time or speaking up about it more than once then their desires are invalidated. This is simply not true, and harmful to suicidal people.
I think maybe your sister doesn't understand this nuance. That when help offered from the medical system is inadequate and doesn't help, it doesn't make the pain of being suicidal go away, it just makes it harder to talk about because people assumes those methods are infalliable.
It sounds like you want someone to sit with you and understand you, with empathy.
