Family members/in-laws won't listen? Please help me! ?

Stop asking/talking to all of them. Go and see a therapist instead.

just dont talk to them about stuff

Go "confide" in someone who isn't related to these people. Of course they don't want to be part of your issues with their own sister. If you're feeling suicidal you need a professional therapist. At the moment you're just coming off as a gossip. 

Parts of this might sound harsh, but I'm trying to help.  When you got to the part about growing up in the foster system, this explained all of it. It really sucks that this happened to you, because it's given you some very wrong ideas.

The biggest problem is you have insanely high (and very wrong) expectations of your inlaws.  It's common to have problems with extended family, but what you don't seem to realize is most sane people want to stay out of it if they aren't part of it.  They even told you this, but you ignored it.  When you say you just wanted support or to vent, this brings them INTO it. 

Same with that stuff about the person who kept cancelling on you.  You made a "stroppy" reply to this and she went to the person she's closest to (her mom).  There's nothing weird here.  Did you really expect her mom to take YOUR side over her own daughter?

Your husband also seems to have some odd expectations.  No matter how close he is to that one brother, it's not the brother's job to be your support system.  It's your husband's job.  Also, friends are always helpful here, but you didn't mention any.

I'm not sure what else to tell you, but I have a feeling if the 2 of you end up breaking things off with them, your husband will start to resent you.  Right now, he thinks he's doing the right thing in sticking by your side, but he doesn't see that it's you creating a lot of this drama.

If you've never gotten therapy for your childhood, this might be a good idea.  It skewed your worldview, which obviously isn't your fault.  But it's on you to fix it now that you're grown.  

PS - If you're the one who gave a TD to Gypsyfish, you're not really asking for help.  You want people who "support" you, which simply means nothing will ever change.  

They obviously don't want to know about your mental health status.  You are responsible for your own healthcare.  If you're depressed, then see a therapist to help you find acceptable ways to deal with depression, anxiety and self-esteem issues.  Limit your conversations with the in-laws as much as possible.  Refrain from "retaliation."

maybe you shouldnt be around them then

You won't like my advice. You don't have to like your family. You can't expect perfect behavior from that- that's unrealistic. And you certainly shouldn't be depending on them to fulfill every need for validation and support. That's what friends are for. You have unrealistic expectation You can't use them as psychological counselers- go seek some professional help for that. But if you interact with them like an adult, and overlook their deficiencies, they'll be there for you in your direst need. Sounds like you've put them off so much with your neediness and your insecurity that you lost that fallback position. Lay off. Accept them as they are-- imperfect. Stop thinking that they should be perfect. 

If you are not getting any support from the rest of your husband's family then maybe you need to cut this sister in law out of your life. You cannot keep subjecting yourself to the abuse because of the way it is affecting you. The only other alternative available is to try and resolve the conflict between the sister in law and yourself. Depending on the nature of the issue, you could try presenting solutions and compromising you. If she is a somewhat reasonable person then I think she would be open to this course of action. All she really needs to do is to see that you are not her enemy. Perhaps getting your husband to help convince her of this may be helpful as well.