It sounds like you have tried to do your best for your sister. You mention your sister has BPD. If that's the case, then your mom's illness and death would have triggered her sense of abandonment. In this past year, it sounds like you have invested a lot of energy to support your sister, and it has worn you down to the bone. You sound exhausted.
You mention you told her you wanted a break for two months. From her perspective, this could have triggered her sense of abandonment again. Remember that pwBPD can't empathize. As such, assume she isn't capable of understanding that you have gone above and beyond to support her. She can't see things from your point of view. Not because she doesn't want to but because she can't.
I am taking a 2 month non contact break from her. I told her about it so she would know what to expect, and kept the focus on myself with "I" statements, etc. Awesome that you used I statements. Did you by any chance tell her that you were taking a break from helping her?
I would probably stick to saying she would see less of you for a while because you have some of your own health issues to look after, and work is taking your energy, and leave it at that. Inotherwords, leave the issues with her out of it. That way she can't take it personally, because if she takes it personally, it's going to escalate the drama. Even if you didn't overtly say this, it is possible she picked up on it because now she is responding by enlisting her son to ghost you. That sounds like an escalation. You mentioned that he texted you to say he didn't want you texting him any more. That's pretty strong language, and you also say it's out of character. I am imagining that you were incredibly kind when you spoke with your sister, kept the focus on you with I statements, and honest about where you were at. If I have this even semi-accurate, it might be the honesty part that she can't handle. pwBPD have
no emotional skin. In my experience, if I need a break from my mom, I don't tell her I need a break from her. I just get busy with other things. She pushes back of course, but then I have to use my boundaries. If mom even suspected I was taking a break because of her, the hate would come out. Once the hate comes out, the hateful behaviors come out.
You mention she has a therapist. This is fantastic. It means you don't need to be her therapist. You are her sister. The roles are different. My mom wanted me to be her therapist. She pressed me for years that it was my duty and I owed her. I never went there. Nothing good can come of trying to play the role of therapist for a BPD family member. Even if you think the therapist is doing a poor job, if you disagree with her therapist on something her therapist said, a triangle is going to start. If you want your life back, it's probably best to step off the triangle. Your sister is an adult, and can make her own decisions, including bad ones. That is my experience with my mom. We can't stop them from making bad decisions. If we try, control issues and conflict will result.
SWOE is the book I started with too. It's a great place to start.
I have had to go LC with my elderly mom. It's been a painful process, because she's quite infirm. But I had to. I am feeling better for it, and getting my life back. I suspect you will feel better after two months too, if you don't spend too much time worrying about what will happen after the two months is over. Many of us suffer from this kind of anxiety.
I have had a T off and on for probably about 15 years, but the last 3 years has been steady and more intense work for me. Many of us on this board have a T. Do you have a T? If not, is this something you could consider? The benefits of a T with experience in BPD can't be overstated.
The educational part of this site is invaluable. There is so much for us (as family members of the pwBPD) to learn, to improve our communication skills specifically with a BPD person, because they are a different set of skills than what we may be used to using with the non's in our lives and communities and work. I have done a lot of work with boundaries, and using SET, asking validating questions, don't JADE. Avoiding the Karpman triangle is another one, which it sounds has been initiated recently with your nephew. All these tools help us, but they don't change the fact that the disease is still there. They just give us skills to better cope from our end. You are probably already aware of these things, but since you asked for suggestions, I thought I would mention them just in case.
I hope you start to feel stronger soon. It is super important to focus on yourself right now, and your own physical and emotional well being. It sounds like you have lots of demands and worries right now. So before you can support your sister (if that's what you want to continue to do), you must first take good care of yourself.

Removing yourself from the drama is a good start. What other things do you find helpful and restorative?