Ending a friendship with my [20 F] “best friend” who is also my SIL [21 F]

I’m in a really tricky situation right now and have been for about the past week and I can’t figure out how to solve it.

Some background:
We were best friends for 2 years, texting constantly all day everyday told each other everything. We would get into arguments but got over them. Until now.

For a while now my SIL aka my best friend has been extremely negative about everything, and texts me every-time she is annoyed. We live in the same state but a few hours apart, but it’s not difficult to see each other. We live very different lives and have opposite goals, which is where I think her and I’s disagreements stem from. We have been friends for maybe 3 to 4 years now, and she was my only friend during the pandemic. I’ve known for years, and my partner has been a witness yo her being very unstable and sometimes irrational. She was diagnosed with BPD right before our friendship started, but is currently unmedicated.

I guess the problem really started a few months back when her constant complaints started to affect me emotionally and I began to feel negative and act negatively towards my partner. I noticed this and thus tried to be more mindful of my interactions with her and how I was feeling before compared to after. I noticed I typically dreaded talking to her, and had a lot of anxiety as well.
This anxiety stems from me trying to give helpful advice when she complains, especially because it’s about the same thing everyday. She wouldn’t like my advice and would get mad at me, and would redirect her anger at me. I began to change how I responded to her by just listening and saying yes that sounds horrible. I can’t imagine. Blah blah whatever. But it didn’t make me feel good. I’m a very optimistic person and she’s the opposite.

Most recently my partner was almost 100% going to lose his job in November. I am a full time student and he is the main provider. This was such a stressful time for us because he’s in sales and he wasn’t able to hit his quota. This was right before thanksgiving. Like I said we live maybe 3 hours away from one another and they wanted us to go see them. The timing didn’t really work out because my partner was working every single day including the night before and morning after thanksgiving. I told her I was sorry, but we were unable to come due to his work at least 2 weeks in advanced and she was pissed. She began to talk about it to my partners whole family and guilt trip us into coming to the point his father called us and told us not to worry about missing thanksgiving.
During what I would describe as the most stressful month of my life she was so negative that I physically could not handle it. I began getting closer with other girls in my classes and formed very fast but strong friendships with them. They kept me sane and I love them all with my whole heart. They were my first friends since Covid and I was shocked by how supportive and enjoyable to hangout with they were.

This is when I began to distance myself from my SIL. It wasn’t like I was ignoring her, we would still text daily. I just wouldn’t be so fast to respond and would really watch my mental health. Not to forget I was also in the final weeks of a college semester. Like I said I would respond to her texts daily or the longest I would go is every other day. I’m not going to lie, I was very bland with my responses and didn’t give much thought to them. Sometimes I would, but I was busy. When school ended I began to really stop talking to her. It would go from every other day to every two days. I just found her increasingly exhausting and unbearable to talk to. She seemed to get more negative. One day while I was at work, I texted her a different opinion and she blew up. She began this entire texting argument (eye roll) saying she doesn’t even know why she talks to me anymore because of my bland responses. I was at work and didn’t have time for this right now so I just said okay. This really set her off, which I understand. She began insulting me and how if I did this to her I would throw a “hissy fit” and how I would “like spend days crying and sending me paragraphs about how ur feelings are hurt “

I responded with telling her “I understand you feelings and apologize my texts were passive aggressive” I then said how I was feeling emotionally drained by her negativity all the time and I was trying to distance myself from her. I reiterated how I loved her and want nothing but the best for her, but I cannot continue to be anxious when I text her.

This is where shit hits the fan. And this has happened countless times in our friendship. She just blows up and says the most hurtful things. In the past I reacted to these texts, but I am very secure in myself and felt no need to respond. And these texts were nasty, she said she gets mad I don’t respond right away and it makes her irritated for when I do respond, I always brag about things (which could be an entire post in of itself, because I don’t tell her anything good that happens to me because she thinks it bragging or gets enjoyment out of my difficult times) how I’m competitive and it’s exhausting, how she doesn’t need to congratulate any of my accomplishments, and then she brings up thanksgiving again. The remaining texts are her just saying how she is supposed to be more important in my life because shes family and the friends I have made are not as important as her. Then in one final long text she negates everything. I was just utterly confused and didn’t respond until the next day saying “Let’s find a time to talk when you get back from your trip. I need time to think about this.”

It has been a little less than a week and I’ve decided I gain nothing from this friendship and I want to walk away. The only complicated part is how do I navigate this as we are family.

I do not want this to impact my relationship with my BIL. I love him so much and he’s like a brother to me. I don’t want him to suffer because of his wife.

TLDR;
My SIL was my best friend and I cannot take her constant negativity. When I tried to discuss it with her, she blew up in my face and I realized I no longer want a friendship with her, but she’s my SIL