Welcome CAMI: I'm so sorry about how you sister is behaving. I can understand how disturbing it must be for you. Did your sister have any of the behaviors, prior to her brain surgery (perhaps less traits and milder)? What was the brain surgery for? Has she seen a neuropsychiatrist or neurpsychologist?
The thread below might be of interest. It discusses the possibility of brain trauma being associated with BPD:
IS THERE SUCH A THINK AS INJURY INDUCED BPD?Sometimes people may exhibit milder BPD behavior that can really escalate during/after stressful events. Brain surgery and your mom having a stroke are very stressful situations.
It will be helpful for you to set some
BOUNDARIES Boundaries are for you and for your protection. It is up to you to enforce them. You may have to to set a boundary about blocking some methods of communication with your sister.
You say she turned against her husband, after the surgery. Has anyone spoken to him regarding the situation. How does she interact with her sons?
I'm thinking you sister may need to be evaluated to rule out some brain damage that could have been caused by her brain surgery or perhaps something associated with the reason for the surgery. Other than that, the best thing for you to do is learn communication skills that can help make things better for you. The only thing you have control over are setting boundaries for your protection and managing the way you interact with your sister and the way you react to her.
There is a wide green band at the very top of the page. There is a "Tools" menu there that has links to some lessons. That can be a good place to start with some communication skills.