Hi Cat,
Many people here can relate. It seems like you have a decent solution to the problem: separating yourself from the daily drama by not reading the texts for a while. If your sibling is lashing out, you are under no obligation to get sucked in, let alone to respond! Besides, if you respond when you are upset, you run the risk of saying something you might regret. And if you respond to your sibling when she's riled up, you're probably only going to add fuel to the fire.
There are a few ways I've thought about these emotionally-charged texts. Typically I take them as a sign that my pwBPD is under a lot of stress, and she's "venting" her issues my way. As she usually vents with meanness and unfounded accusations, I know perfectly well that I had absolutely nothing to do with her current distress. Since I'm neither responsible nor to blame, I don't take offense so to speak. Nevertheless, it's hard to witness someone become unhinged like this; it's hard not to worry. If that happens, sometimes I give myself a break, by not reading the correspondence right away, let alone reply. My opinion is that she doesn't want a reply anyway, not really, because she's unhinged.
Normally, what I do is wait. I think of this as giving my pwBPD an "adult time out," i.e. time and space to cool down. There's no clock on this. I wait until she reaches out again in a civil manner. If weeks and months go by, then I might lob a short, friendly greeting, something like, "Hope you're enjoying the summer!" I don't ask anything from her though, so she doesn't feel like she's "forced" into a reply.
In the meantime, it might help to think of her messages as spam, because they ARE spam. Why? Because spam is designed to convey false urgency, and to get you to respond ASAP, typically in a way that you wouldn't do otherwise, because you feel undue pressure to act. You know what you're supposed to do with spam, right? You're supposed to delete it. So go ahead and delete it.
Another way to think about the situation is to ask yourself, do you want to ride the emotional rollercoaster? Just because your sibling is riding the rollercoaster, it doesn't mean that you have to ride alongside her. Instead, you can choose to wait patiently at the exit.
By the way, I generally don't "do" texting, and certainly not as a way to convey emotionally-charged topics. Ditto social media. Instead, I tend to talk on the phone. Like another poster said, it can be easier to control phone conversations, because you can hang up if things get too tense or hostile. In addition, since the conversations are live, I think they tend to be more considerate, as well as focused. With family members, it can be easier to establish a routine for calls, such as once weekly, while driving home from work, walking in nature or folding laundry. That way, you stay in touch, but you're not subject to daily dumping and venting. Sure, there's some venting, but it's limited to the duration of the call, which is under your control. And if you combine a call with another activity, like laundry, you have some distraction that helps ratchet down the emotional intensity, as well as give you a stopping off point (oh, gotta go now, talk to you next week).
Having written all that, you might need to train your sibling how you want to be treated. How? By establishing firm boundaries. One technique I've used is what I call the three-strikes rule. If my sibling or pwBPD is talking about an overly negative, stressful, irrelevant or hopeless topic that stresses me out too much, I'll say something like, "This is stressing me out / we're not getting anywhere, let's please change the topic." If they say, "Yes, but . . ." I'll respond with something stronger: "If you keep talking about this/if you keep yelling at me, I'm hanging up." That's strike number two. If they're really stubborn and don't listen, and they continue to vent or yell, then I'll say, "Gotta go, bye."
