My relationship with my sister is strained.
I've commented on threads over the years about it.
But I learnt from a very young age to passively disengage and have very limited contact.
Interestingly my cousins and friends actually have over the past decade or so spoken out about how they hate the fact she's allowed to be so rude to me and I'm not allowed to comment.
My DS (14) finds her very difficult to be around as she also makes comments to him.
She's clever and sly to the point she can make out you are in the wrong even when she directly insults you in front of an audience.
Today we were at a extended family BBQ. Ds was due his medication. It's in a small box that isn't transparent so it's just a box iyswim?
I went inside where he was and said it was due and to go and get it. I was preparing the drink in kitchen. I'd called him to me and said it quietly so no one knew what he was doing.
Ds walked to where I'd been sat in garden and got box out of backpack to bring to me.
One of my cousins joked asking him if he'd bought his own packed lunch too. (Hes an athlete so it's a standing joke how much he eats compared to how thin he is!)
Ds just said "no it's my medicine"
So my sister then starts (loudly) telling him off because he shouldn't be getting it out in front of everyone and making a fuss about his medical problems and why does he need to make such a fuss.
Literally the 4 people sat there were the only ones who'd seen him and then about 15 other people's attention was drawn to him.
He was embarrassed and upset. He has ASD and was totally confused by what he was meant to have done.
To lighten his mood I just joked quietly (my aunt stood next to me didn't hear) that he'd done nothing and he should be proud it had taken 2 hours to wind her up today rather than 2 minutes 
It all sounds so bloody ridiculous written down and it is as embarrassing to be there as I'm sure it sounds.
Literally it should have stopped at "my medication" when only 2 people who didn't know would have known what was in box.
There must be a way I've not yet learnt to stop this. I don't want ds being upset or becoming her next target and victim as I know how it feels.