"Cousins are very strange beasts. Be careful."
I can second this, from personal experience. When my grandmother died, my younger cousins - whom I'd had a distant, but generally okay relationship with up until this point - actually prevented me from being able to attend the funeral. Death threats were issued by one of them against me, my parents were dragged into the situation by them and manipulated, and my uncles decided that if I showed up, then they'd "remove me, themselves". My crime? My grandmother left me a sizeable amount of property/money (which I haven't received, due to my uncle's shenanigans) and I was a single parent. My oldest was 2 when my grandmother died, she's now 23... and ever since the day my grandmother died? Not one member of my father's extended family has acknowledged her.
I didn't attend the funeral. My parents went, and played happy famillies - and that was their choice/right. But I stayed away. Not because of how I'd been turned on, but because I didn't want my grandmother's memory to be disrespected by ugliness on a day that was/should have been all about her.
Unfortunately, OP, some people insist on making funerals, other people's weddings, baptisms and the ilk all about them. How they hate, don't like, never took to someone else in the congregation. That's their choice. But you can make a different choice. You can rise above it. You can ignore your sister, completely, even if/when she starts a scene, or tries to engage you in conversation - because that's what NC is about. But if you do, make sure it's tactful and not designed to cause a scene... because that may be what your sister is hoping for. If she's posting all over social media, then she's seeking attention, or validation, to appease her own guilt at not having been a constant presence in your grandparent's life. That's her choice/right. But you don't have to validate it. It's meaningless trivia to you... right?
Rise above, OP, and I'm very sorry for your loss 