Trying to build a relationship.
I’m sorry that you went through that, particularly at the hands of your sisters. Many people get bullied as children, but it is so much worse when it comes from inside your house from your own blood, not only do you not have a safe place to go but it also leads us to internally question what is wrong with us if even the people who are supposed to love us act that way. Unfortunately, this type of behavior has nothing to do with us and everything to do with them, but we still have a hard time believing that fact when it feels so personal.
It’s great that you have found a lovely warm person in your third sister, but a shame that she hasn’t had more to do with your life. In this instance, you have a huge bearing on how much she has to with your life moving forward. I would suggest reaching out to her via Facebook and suggesting a catch-up at a quiet cafe. I think you should be honest with her at this catch-up about the horrible time you’ve had with your sisters and that she has always had a positive effect on your life and that you are looking to have more people like that in your life. I think if she is a warm, decent person as you seem to think she is that she will of course love to have you in her life more. The only trouble with people I find, myself included, is that life sometimes gets in the way and you don’t catch up with the people you want as much as you should. But I think kind, caring people are worth making the effort for.