My four siblings and I grew up with the same parents and almost identical upbringing, aside from the fact that my father was extremely emotionally abusive to me from the age of 14-18. Only my younger sister was ever aware of the way he behaved towards me, but our older siblings remained oblivious because they were often out and about, left home before us, and he’d refrain from screaming at me and belittling me when they were around.
This led my older siblings to believe that I was an irrationally moody and sullen teenager with “behavioural problems” of my own making. They blamed me for my eating disorder, saying I wanted attention, while my dad regularly told me that I was disgusting, fat and lazy, even though I was underweight. They told me I was a bad daughter and should be more respectful of him, while he would tell me that nobody could ever love someone as pathetic as me. They thought my self-harm was another attention seeking method, while my father put immense academic pressure on me and repeatedly tried to convince me that I would fail at life and end up on the dole, despite the fact that I achieved good grades and was a model student.
My older siblings never knew about any of this until years after the fact, because my dad hid it well and I was too ashamed to tell them. Strangely, I also wanted to protect them from the fact that he could be a bad father, because they loved him and though the world of him. I didn’t want to taint their view of him. Even to me he would behave like a great father infront of others, and I never wanted for anything except his kindness, affection and approval. He gave me money for whatever I needed. I was always fed and clothed. He paid for school trips for me and supported me financially while I was at university. But even so, I wanted to die everyday because of the way he treated me while I lived at home. He had destroyed my self-esteem, self-worth, trust and perception of healthy love.
For several years afterward I distanced myself from my father, which my older siblings criticised and didn’t understand. Luckily, the unwavering support of my younger sister reassured me that I was not crazy or overreacting, and I don’t think I could have got through it all without her. I managed to undo years worth of emotional damage with a lot of councelling and soul-searching. So while my siblings frowned upon my decision and thought I was being petty, I know that I would be dead by my own hand right now if I had forced myself to maintain a relationship with my father at that time.
I guess my point is just because you aren’t aware of any significant mistreatment, doesn’t mean there wasn’t any. And while you should not get involved in the quarrel between your parents and sibling, you should still maintain your relationships with both unless there is some real reason you can’t. Because otherwise she may feel completely alone.
By no means am I suggesting that what happened to me is exactly what has happened to your sister, but there may be more to the situation than you know. Or she could be overreacting. But you don’t know, so proceed with empathy.