CHChuck, what a terrible thing to go through! It is indeed a huge betrayal and I feel for you in this situation.
All the same, I would not recommend letting her know that you know. Of course secrets between spouses is never a good thing, but sometimes we have to ask ourselves why we want them to know. Is it to relieve our consciences or because it is a way to address an issue that hurts us? Bottom line, she trusted you by letting you use her computer and you betrayed that trust. Something like that with a BPD person never ends well. It is a disease where trust lies at the very core of healing, and your every effort should be directed to building that trust.
It is a feature of BPD to try and build a support network independent of their spouse. I used to feel very threatened by this, but the more I learn about my beloved's history of broken relationships and emotional betrayals, the more I understand the urge to build a fallback position. Indeed when she dysregulates I perfectly understand the temptation to walk away... So I strive to be patient with her need to have others in her life, even if her way to get their support is over my metaphorical body.
What you might find helpful is this discussion on how to avoid emotional triangles:
https://bpdfamily.com/content/karpman-drama-triangle It saved my bacon during a very tough time and I certainly credit it with my current relationship peace (no doubt short but lovely while it lasts
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If you feel a need to speak to her about her brother, then wait for a calm moment and refer to him without mentioning the texts. I agree that hostile in-laws make a difficult situation worse, but two wrongs will not make a right. It is better to forget as much about the texts as you can and focus on the timber in your own eye.
Khib
