Hi
naiveoptimist and welcome to the site. You are in the right place to get support and to talk with people who get it. Oh and: Congrats on the birth of your baby!
I undertand your concerns about re-establishing a relationship with your sister. It is a difficult choice and one that is very personal. There is not much you can do if she is not responding but what you can do is keep working on yourself in therapy and on this board. That will allow you to decide what you want and determine the parameters of any relationship that may be possible for you.
People with BPD do have trouble with empathy in that they may have it but what they do with it or how they handle it is very dysfunctional. It is sometimes too overwhelming for them to handle and/or it may be felt as a threat to their own self and they can sometimes lash out. It is very hurtful and frustrating to be on the receiving end of this.
I feel like I'm being selfish in putting myself and my family first, but I just can't find the will to start a relationship with her again.
It is not selfish to protect you and yours from harm or undue stress. Nor is it selfish to expect your sister, regardless of her disorder, to be responsible for her own well being and to manage her well being responsibly disorder or no disorder. A good thread to read is
The Do's and Don'ts in a BPD Relationship . it outlines what is needed to have a successful relationship with a pwBPD (person with BPD) and it can serve to help you set realistic goals and expectations as you decide whether to re enter any relationship with your sister.
I fear being pulled back in completely and don't know how to stay strong.
We have many articles on topics ranging from self care (for you) to communication strategies and understanding the behaviors in a pwBPD that will help you to not fall back into the same old patterns with your sister. I am glad you posted and reached out for help and as you jump in and post more we can better direct you towards speific tools that can help you.
Good to have you here.